Re: Upgrade question:)
Get the fastest spindle speed possible (7,200RPM or 10,000RPM not 5,400RPM) and the biggest cache buffer you can get (you want 32MB or 64MB) and if your motherboard supports it then try to get a hard drive running on SATA III 6GB/s, otherwise any SATA II HDD will do. Happily most modern day hard drives have pretty much everything I listed here so just the one with the most space for you.
My Custom-Built PC
Core i5 760 2.8GHz, 8GB DDR3 Dual-Channel, 2TB HDD, 120GB Crucial M4 SSD, EVGA GeForce GTS 450 SC, Foxconn H55M-S Motherboard, NZXT Lexa S Case, CIT 650W Neon PSU, Windows 7 Ultimate x64 SP1