If you want the new PC to be quiet you have to be VERY careful about your choices in:
1: Graphics card, This is your main screamer. Find one with a cooling solution that is relatively quiet, or get stock and buy a 3rd party quiet cooler option.
2: Your case and CPU heat sink fans. The noctua HS fans that come with that CPU cooler are good. I how ever don't know the case fans you are linking, go for the SPCR tested and recommended fans like Nexes, Noctua and Scythe. http://www.silentpcreview.com/article63-page1.html
3: Why are you putting those 3x500GB HDDs in the new comp? You need the space? You're going to use the old PC as a server, why not keep those noisy drives there and out of your PC room? My main PC is pretty quiet, but only when those damn HDDs spin down -_-
The X-750W is more than enough for what you are aiming for, got one in both my PCs.
i5 vs i7, you need the Hyper Threading? Using programs that use it properly? If no, go 2500K
1 playing around with switching stock to custom cooler is out of scope. don't have time for this. maybe passive powercolor with strapped fans or the msi i mentioned would be best choice.
2 Noctuas are pretty laud at 12v but at 40% with speedfan noise is gone. Will consider other fans from the linked article.
3 those are drives i move from old pc 2x500 + raid mirror on two other 500. Will move one 500 to the server along the 2T drive i was using there
I think HT is not so crucial. Compiling 1s longer doesn't hurt
If you'd choose a Radeon 6850, and your budget can be stretched a bit further, then I'd suggest you buy a passively cooled 6850 and then strap on a very quiet (i.e. undervolted) cooler on the heatsink. In that way, you are able to control how loud the fan is. This setup also show very low temperatures at load.
good option for <100$ more if you count the additional fans cost i think
think i can afford it