If you're an AMD guy and want this PC for gaming, look into the FX series for a processor, they are top notch gaming processors from AMD, but if you think you may be doing a ton of multi-tasking and such go with an X2.
Yes, you should go with the new AM2 socket if you do choose to go AMD, considering it allows the use of DDR2 where as none of the other AMD sockets allow this at the time.
I don't know much about the Core Duo but I do know it will cost a "pretty penny" so if you have a lower budget I would probably stay away from it.
If you are building a machine soley for gaming of course you are going to want to focus on a strong video card. I know that the Radeon x1800xt 512 and the x1900xt 512 are both great cards as well as the Geforce 7900gt and the 7900gtx (although quite expensive). If you are looking to keep yourself new for the whole next gen of gaming you are really going to have to look into a "beafy" card to hold you over for such games as UT 2007 which will require a heaping helping of a system to run it maxed. My suggestion here is to look around in your price range at high end video cards. Get the best you can afford if you want to stay new the longest.
As for RAM, I would look into getting 2gb since RAM is coming down (slowly) in price of DDR2 with some low latency such as the Corsair XMS2, OCZ, or the Crucial Ballistix if you really want to have a booming system. Again, shop around until you find your right price range and best bang for your buck.
Motherboard is strictly up to you. A DFI Lan Party is nice, any ASUS board is nice...just look through for good price, features, and compatibility with whatever other components you pick out.
Hard drive is also up to you, although the new 150gb Raptors are very sweet (but they will cost an arm and a leg + shipping). Again, your choice, you don't have to be too picky here.
Well, that appears to be it except maybe a DVD burner and some other addons you can find yourself. I could have sat down and made out a list of parts but giving advice from my experience and allowing you to apply your own skills with what you now know will help you in future builds.
Hope that helps!