I realize that I will have to make a decision regarding Vista vs XP with respect to memory availability in order to hit the full 6GB.
Actually that isn't Vista vs XP. That's 32-bit vs 64-bit, both XP and Vista (and Windows 7 which you can download now and use for 1 year before you buy Windows again!) will work fine. I have 12GB of RAM in my 64-bit XP box for example, and I'm installing Windows 7 on a different hard drive in the same PC at the moment.

That said, I dont have experience with that CPU or GPU but they should do just fine.

Edit: Gotta say, the difference in performance between XP and Win 7 is night and day -- huge performance increase for me. Highly recommend Win 7.