For a GPU, I'd suggest a NVIDIA GeForce 7600 GT (or a GS if you want to save some money). If you have an even tighter budget, goto NVIDIA's site and look at the vista capable GPU list and choose one for your needs
For a mobo, I would like to say that a 975 series mobo by ABIT is good but that again is quite costly I believe.
If you really want a processor, E6600 wins
but that isn't cheap either. It comes upto what you want and what you can afford. Thats all
I dislike Pentium D's because they are midrange step up processors to the core 2 duo's imo. Conroe > *
And oh, 1 GB of ddr2 ram and you're all ready