You definitely need a good PSU for that system, I'd get an Enermax Modu82+ 625W, that leaves you some headroom and gets you one of the most stable and quietest PSUs out there.
Other good brands are Corsair, Seasonic and PC Power & Cooling.
The system should fit fine into most midi towers, i quite like the Lian Li PC7 or PC6x cases:
Newegg.com - lian li, Computer Cases
Pick a midi tower case that you like^^
For a hard drive I'd get a Samsung F1 320GB, the WD 320GB are somewhat slower and I don't know about Seagate with all the failing drives from them around atm. 500GB models are a little bit slower since they use two 250Gb while the 320GB models use newer higher density 320GB platters which makes them a little faster (and quieter: less platters usually means quieter).