I have one of those pcmcia cards with the ports built in, and i had it jacked into a friends broadband adapter. you don't HAVE to use DHCP, and as far as the amiga's concerned, i'd avoid it as most the time the amiga's IP stack isn't clever enough to understand whats going on anyway.
just make sure you have the cnet.device package. its either on aminet, or on one of the old amiga active CDroms.
set it up with a static IP address that is in the routers IP range, but with an IP address that won't be taken by another machine. (something like .100). and set the gateway as the routers IP address, and you should be able to surf the net, telnet, whatever on the net just fine. no router reconfiguration required.
having a static IP is good for samba configuration too. i did have a little smile when i saw my A1200 in my mates "network neighbourhood" on his PC (need samba for that) and he mounted a couple of my drives as network drives. most cool.
trouble is, i don't really like miami. its rather slow for an IP stack, and it gets a bit annoying when the tcp programs are called "miamiping" and stuff. half the utility prorams for TCP/IP are missing, and paths don't get set etc. much better bet is to get the netconnect3 package from vapour - that'll have all your web browsing, IRC, chatclient, ftp, you name it, it's there internet requirements in one handy package, or 3.1roms and OS 3.9 with a cut down version of genesis (that doesn't time out after 30 minutes! grrr) miami is good for testing though. least you know it works.
(just gutted i lost my netconnect2 key plugging an OS3.5 disk into an OS3.0 machine error, error, error, error, error, error, error, error, error, etc.) :-)