I have a Vertex II in my PC (C2D E7200) and from what I can tell, the real bottleneck is the CPU. When a program/game is loading, for example, decompression of its data and realtime virus scan become the bottleneck. You can tell by looking at the HDD activity indicator LED, it's far from being full-on (indication that the PC was waiting for the drive).
Then I have a new laptop with a comparable CPU (i5 540M) and Intel G2 drive and the real world performance difference between the two systems is virtually nonexistent.