Hi Oria,
With pc's these days, pretty much anything is capable of doing a job in terms of raw power. I think more essentially with a main studio PC is that it be quiet. Making PC's noiseless but powerful is a different and more expensive beast. I'm sure things will have moved on since we built our workstation but my recommendations would be -
Quiet drive enclosures (
http://www.quietpc.com/gb-en-gbp/products/harddrivesolutions/quietdrive)
I've got these in both studio pc's on all drives and the media machine. I can't hear any drive noise at all unless I'm an inch away from the pc. Better than the ones I've had in the past and a good thermal transfer... A MUST!! I recommend the Samsung Spinpoint drives as these are naturally quiet anyway before going into the caddy. SSD are still too expensive for the amount of storage, soon though... soon.
Quiet Fans (
http://www.quietpc.com/gb-en-gbp/products/casefans/s-flex120800)
Same with these... where 120mm fans are possible. I have these on all pc's again. There is quieter available but for the price break to noise ratio...
Also with fans, you can get brackets to mount on the PCI slot screws that will position a fan directly down onto your motherboard, coupled with a near silent fan inside the case it is quiet. I find this a good option for passive cooled pc parts as you lose the noisy small component fans and cool all with one big quiet one.
Processor cooling - too many options!!
I've always gone for Zalman... everytime! Fan coolers are very quiet but I have reserators for the 2 studio pc's. We have 2 on the main pc and one on the 2nd pc, there is a tiny amount of pump noise when your head is close to them but quieter than a fan due to the pumps being fitted at the bottom of the water chambers. Personally I would always go for intel processors, they seem to always have the edge over AMD for media work.
Case
So many options, kits and fancies out there... just try to get a solid metal case. You can buy acoustic kits and neoprene feet for it after, the main thing being solid. Cheap cases can almost become like a snare when rattled. Plenty of room for expansion with plenty of 5.25" bays for quiet caddies. Consider an external DVDR drive instead of internal if possible as most disc drives have fans built into them, plus you can turn it off when not needed.
Hope that helps... as you can see, quietpc is a good place to have a hunt around, although price check (bear in mind shipping!). Manufacturers... Zalman and Scythe I would highly recommend.
Peace
Lee Fury & JTB
http://soundcloud.com/biteandgouge