what is the best way to make my Pc quiet. I haven't ordered all the parts yet..
Chose careful your components, go for parts that already known to be quiet, add the least amount of noisy parts, like fans, hdds, dont have open vents or places where noisy might get through.
2500K i5 intel
gpu- radeon 6870
Great CPU, among the best offer atm for gaming and ocing, the gpu is decent mid end, some users have flashed their bios to allow lower rpm at idle. Search here for 6870s see what users have done with them. If not you can always get an after market cooler and strap 120mm low rpm fans.
psu - 650 watt. (any psu to recommend for quiet sound?) Seasonic X-750 $130 free shipping
is at good price atm, it should be passive at idle, and very quiet under load.
Cpu cooler? or the stock intel ok?
This is varies from each person, to me no, but for a lot is fine, my suggestion is to try it first see if you like it or not. You can always get an after market cooler later, a cheap one is Cooler Master Hyper 212 $20
or if you willing to spend a little more, Scythe Mugen 3 $49
comes with very decent PWM fan that can drop to 300rpm via BIOS/Software (depends on your mobo).
stock fans.. or should I use custom fans?
Try the stock fans first, R3 has a fan controller that you can use to lower the rpm of the fans to your personal liking, and see if the performance of the fans is good enough for you, if not just add more fans.