[KarmaKiller]

Well really depends on your budget. The fastest drives you can get atm are SSD's, but IDK if your willing to spend that much on one. They don't have a lot of storage space for the price, but even a normal HDD RAID setup normally doesn't beat them in performance.

IMO, a great mix of speed and price is probably the WD 640Gb black editions.
Newegg.com - Western Digital Caviar Black WD6401AALS 640GB 7200 RPM 32MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive - Internal Hard Drives

If your looking to spend more, or maybe get a little jump in performance from that, then check into a Velicaraptor
Newegg.com - Western Digital VelociRaptor WD3000HLFS 300GB 10000 RPM 16MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive - Internal Hard Drives

But for that price, it's hard for me not to say just get a SSD.
Newegg.com - OCZ Vertex Series OCZSSD2-1VTX120G 2.5" 120GB SATA II MLC Internal Solid state disk (SSD) - Solid State Disks