From testing, there's not much difference shown but a few frame increases. To me it seems that you'll notice a difference when you have a lower end card, because there is a threshold in frames per second that you just simply won't realize a difference anymore.
You will notice a big difference between the FX 5200 and the 6600GT, and even bigger one if you get a 6800 series.
ASUS A7N8X-E Deluxe
AMD Athlon XP 3200+
OCZ 1GB Platinum PC3200 Rev2 Dual Channel
ATI Radeon 9800 Pro
Enermax Noisetaker 485W
2 x 80GB SATA Maxtor Diamondmax 10 on RAID 0