How can someone be sooo stupid?

[Floid]

Well, I gave in and registered, just to respond to this.

First off, yes, those are two ethernet controllers; presumably this is some sort of high-end hobbyist/workstation board.  A 'PHY' refers to the 'PHYsical' transceiver, so seeing two specified means that each controller should, indeed, terminate in an ethernet jack you can use.  (It's not entirely uncommon to see boards made ignoring/supplanting integrated features, like the NForce's MAC.)

Now, if you're using Windows, and have a working driver for your particular flavor (which *has* been a problem for many),  the USB modem will work fine, though you won't have the flexibility to plug it into a cheap Linksys router.  The problem is not so much that "USB sucks" or "USB DSL modems suck," but rather a general lack of understanding/support for the standards involved.  If I can talk out my posterior for a bit (read: I don't know for sure, but I'll pretend I do; enlightened ones, please correct me):

-DSL itself runs over a link-layer protocol called ATM, 'asynchronous transfer mode.'  You could conceivably do all sorts of wondrous info-superhighway things with ATM (traffic prioritizing for voice-over-DSL, digital TV, etc), but in practice, you don't, and it just encapsulates the PPP, IP, ethernet, or PPP-over-ethernet packets that make up your nice fast internet connection.  Your provider likes ATM because they've been using it for their voice traffic for years, and it keeps things managable for them.

-When you have a "normal" DSL->Ethernet bridge ("DSL modem"), the box is "forced" to peel off the ATM layer, and do whatever may be necessary to spit ethernet out the other end.  You might get left with a headache like PPP-over-Ethernet, but those are still plain ethernet frames, and just about every platform on the planet knows how to deal with them.

-When you have a USB modem, however, it's likely to be spitting out the ATM cells (packets, frames, whatever you call them) directly, since there's no reason for it to 'convert' to ethernet (and ATM does, theoretically, have features that ethernet doesn't).  This would be cool, except now you're faced with a more 'arcane' technology that the fresh grads from Bob's House of Visual Basic just hired by DSLModemco might not fully grok... and which alternative OSes - even Linux (http://linux-atm.sourceforge.net) - aren't likely to support off the bat.  Add in some of the vagaries of USB itself- it's just becoming a mature technology- and you can see why people whine.

That situation was more dire around the launch of XP; nowadays, I think most companies' drivers have stabilized.  I do believe there are a few USB modems out there that dump ethernet frames, rather than ATM, but it all depends on the technologies your provider is using, and even then, don't expect those to work easily under free *NIXes until/unless someone hacks the device codes into the appropriate places.

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Basically, from my POV, whatever you have will be obsolete in 5 years.  I'm guessing it'll be about 3-5 for the USB/ATM standards mess to truly stabilize to whatever PnP nirvana it's supposed to be, across the board... though as I've said, it's probably almost there on Windows, barring new version jumps.
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That out of the way, you have two ethernet ports (and with a USB modem, you'd only need one, anyway), so you *could* get by with a hub and Internet Connection Sharing, rather than a dedicated router.  Whether you'd want to put yourself through the hell of securing/relying on Windows is another matter, but 2k is stable enough for the task, barring  worm attacks, ActiveX-induced trojans, mail virii, security-patch-induced-holes and so forth.

You probably *would* be happier with the ethernet modem and the Linksys, if for no other reason than everyone else on earth having that configuration.  I just wanted to explain *why* the USB situation sucks; it's basically a bad interaction of good technologies, and now you can explain that fact to others.  

[Floid]

DSLreports/BroadbandReports is the place to dig, of course.  I'd imagine you could request an ethernet modem from even the most braindead of ISPs, though they might hit you the $200 equipment charge for it.  

See if they have OS X drivers for the USB modem, and if not, perhaps complain that you made the 'switch' to an iBook.  If they offer Mac support at all...

Running Linux, you could definitely set up a 'NAT'/'ip masq' setup, but if you expect to reboot often, the dedicated router's the better deal.

One product line to look into might be the 2Wire Homeportal, and BellSouth is actually a partner, so it should Just Work:  2wire.com ... You should be able to acquire one off eBay inexpensively; find a model with a DSL modem built in - product matrix and whatever features you might want.  They take an "activation code," which looks a little scary, but is actually just a hash for the settings for your ISP (meaning they just hand them out off the 2wire support site, it's not any sort of resale-prevention scheme).  I mention these mostly because of the partnership deal; I happen to be using one of the modemless models myself, and it's been "pretty okay," meaning slightly less braindead vs. Linksys, but still no panacea.  You get the added options of HomePNA phoneline networking and possibly wireless, which can come in handy, if you have hardware to use it with.

(I should note that 10mbps HomePNA 2.0 is nifty, but requires either Windows or $60-$150 bridges per room you want to use it in.  1mbps HomePNA 1.x is less nifty, but can be done with eBay $6 PCI cards that work under Linux, at least 2/3 of the *BSDs, and probably Amiga if you sent a card to the right person.

I should also note that I actually *couldn't* get my 1mbps card going under the 'Knoppix' Linux DemoCD, even though it's supposed to be Linux-supported, and FreeBSD handles it fine.  That was a fairly obvious bug in the driver- it recognized the card, but didn't initialize it properly- and I can only guess it was a one-off bug in the particular kernel version used.)

[Floid]

I'm more the BSD type myself, but a dedicated *NIX box (even an old 486) is always a good way to do it.  If you'll be booting in and out of Windows often, it's probably not the way to go.

I have no idea about the Cayman, but DSLReports would be a good place to ask, and you might find someone familiar with both it and the 2Wire; I'm guessing it's another "perfectly decent" product.

The advantages of the 2Wire would probably be the option of HomePNA, the firewall controls (their system presents a menu of common games and services, updatable from their servers/with room to add 'custom' items, and seems to be based on IPFilter beneath), and the local DNS service (which I've just confirmed is as easy as setting the 'host-name' option in a machine's DHCP client).  One-click updating of the firmware from the management page is also fairly useful; AFAIK, at least Linksys doesn't have that, but I've never been very intimate with one of those, either.  Also the eBay $80? pricepoint.

Think that's about the extent I can give..  I have to say,  local DNS is a *very* useful feature to have (and unfortunately, BIND is a buggy pile of bits, and djbdns is a tad braindead in its own way, so it's nice to see it in the dedicated appliance)... only reason I hadn't explored it on my setup until now is because I'm actually set up out of my uncle's basement, and there weren't enough boxes to make it useful.  Once you get some extra hardware on the LAN, it's a lot more fun to type 'ssh miggy' than 'ssh 172.16.0....  wait, DHCP refreshed, let me walk to the box and check ifconfig...'