With the price of AMD chips, the 3.0 or 3.2(if you can find it) chips are a great buy($100ish). The Intel quads seem to be better than the Phenom, even tho the Phenom is a true quad. But thats just what I have read. I do have a Intel Q6600. Excellent chip.
With the price of HDs, a couple of 500G might be a better storage/cost value. With a Antec 900 a stock cooler shouldn't be a problem. As far as video cards a Antec will take a full size card. I run a 8800GTX on mine with no problem.
I use the OEM/Builder OS's in all my systems. Not a problem. Change components, you do have to re register windows. (probably same with OEM or Retail) I am waiting on Win 7. Until then I am using XP. MS will stop supporting XP in 2010. (?) Vista with SP1 update isn't that bad. Just not worth putting money into as long as XP is supported and a new better OP is just around the corner.
For my AH rig I like to keep it simple and basic. Dual core,Good MB, 2GB Ram(atleast)1HD, 1VC,1DVD/CD and adaquate cooling.
My AH system; Win XP Pro x64
Modified Mid tower case(10+ yrs old)
Giga EP45-UD3R
Intel E8400
8800GTX VC
2GB 800Mz Ram
10k WD Raptor(150GB)