Here's my computer history in the last 7 years...
2002 - Intel P4 Northwood (XP on 2x80GB RAID0)
2005 - AMD X2 4400 Athlon64 Toledo (XP on 2x74GB Raptors RAID0)
2007 - Intel Core2 Quad Q6600 Kentsfield (Vista on Areca 1210 w/4x74GB Raptors RAID0)
2009 - Intel Xeon W3540 Quad Bloomfield (OSX on 2x80GB X25-M SSD RAID0)
The biggest improvements in performance were from the single core to dual core, and then mechanical storage to SSD's.
BTW, that 2005 build was a rock-star, it could still be doing HTPC duty today if I hadn't abolished all Windows computers from my household and replaced it with a Mac Mini.