I built my latest PC the other week, it consists of:

What I bought:
motherboard: GA-MA790X-UD4P  <-- takes DDR2 ram
processor:      AMD Phenom II X2 550 (dual core but I was able to unlock the other 2 cores)
RAM:                2x2gb (PC2-8000) Mushkin Redline 
GFX:                 XFX 5750
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What I had:
RAM:               2x1gb (PC2-6400) Geil 
sound:            X-fi extreme music
case, mouse, keyboard, etc...
 
The 5750 should be decent. The cpu is nice, I got lucky that it could unlock all 4 cores. You need the proper motherboard and some luck that you get a cpu that has two disabled working cores, and not two disabled busted cores. The 550 is said to be a quad core, that has two disabled cores and is sold as a dual core.
 
I installed Win7 Pro x64