I'll throw in my 2 cents as well even though I haven't posted much here, as I am trying to absorb as much information about Amiga's since I haven't been around them since I was 12. For you questions about processors, it really depends on what you are going to use the computer for.
If you have no intentions of overclocking your processor then I would recommend AMD. They are indeed slower than Core2 and Core2Quads, but they are considerably cheaper, and if you hunt around you can get the EE (Energy Efficient) which depending on the model consumes from 35 to 65 watts. Keep in mind that if you don't buy a black edition of an AMD chip you will not be able to overclock it very easily or far. AMD still uses the 65nm architecture which isn't super energy efficient. Most Core2Duo chips are 45nm and are very energy efficient. They also overclock really far, and stay very stable. Also about Core2Quads, from what I understand is that they are two Core2s with an interconnect bridge. If you want a true Quad core from Intel, wait for the Core i7. These are true Quad cores from Intel, and they are incredibly incredibly fast, somewhere in the range of 25-40% faster in certain apps than a core2duo. However, they are very expensive, as I believe I read that the entry level i7 chip is $250 USD. There is only one chipset, the X58, currently that supports the i7 as it goes to a new socket called socket 1366. It also sports an integrated memory controller like AMD CPU's do. The motherboard is as much as $300 for the i7. If you are using programs that utilize more than one or two cores, then definitely go for a quad core. However for gaming, most games work with two cores, but show very little gain on quad cores, many times being outpaced performance-wise by a faster clocked dual core. When it comes to memory, I cannot really state whether you should get DDR3 or DDR2. DDR3 does run slightly faster, but can be in the range of 50% or more expensive than DDR2. If you go with a core2duo, you can use either DDR2 or DDR3. Looking at Newegg, 4GB of DDR2 1066 can be gotten for as cheap as 79.99 where DDR3 1066 will run at least $100. Note there will not be a speed difference between the two, as the latency on the DDR3 is considerably higher. A happy note is DDR3 runs at a lower voltage, requiring less power. Note that AMD does not support DDR3 yet, as that won't be supported until their next line of processors comes out next year for a new socket called AM3.
For video cards I would recommend looking into AMD/ATI's line of 4800 series. They are a very good price to performance ratio, with the 4850 taking the crown for price/performance ratio. The card is exactly the same internally as the 4870 except it uses less, slower memory and the core is clocked slower. You can find a 4850 for as cheap as $130, as that's how much I paid for mine. It's a Powercolor. Still, even though it's slower than the NVidia series of GPU's, it's also very inexpensive. I can run Crysis Warhead (sorry don't have the first one) at 1280x1024 with the graphics set to enthusiast and still get about 30 Frames per second. I run Dead space maxed out at 1280 by 1024 (that's the highest resolution my monitor does) and it runs super smooth and is creepy as hell. :-)
I use Gigabyte motherboards as they have worked best for me over the years. I haven't run into compatibility issues with any components I use, and I always use a added in Sound Card. Currently using the X-Fi Fatality Professional PCI-E card. Very nice card. This same card caused me to purchase a new motherboard a while back as it caused my whole system to crash as I wanted an AMD chipset for my computer and the only company that had one was ASUS. That motherboard made me want to buy a Mac!
Bottom line really truly is, buy for what you need and be certain to leave what room you can for expansion, be it a faster cpu, or video card, that way you don't have to rebuild your system again in six months, cause that always stinks. :-D
I guess that's more than my 2 cents eh? Sorry for the ramble. Have fun building your new system!!!
