The CPU will limit the FPS, but the better card will still show a good improvement, especially when the difference between the cards is so great.
I cite a relative's case where he recently installed a 9800pro on his Athlon 1.2ghz machine.
He scored in the high 5000's in 3Dmark '03 with THAT cpu....... I was surprised, as was he. I'm running a similar powered card and only get just over 6000 3d marks with my overclocked 2800+
However, any time the CPU is overloaded or inadequate, you will see the effects of the shortfall, on any system.
I'm running IL-2 Forgotten Battles at the moment, and in Perfect video mode, my FPS drops like a lead balloon, which is terrible because the water rendering in perfect mode is just awesome.
I asked about this problem, and got a few replies saying that it wasn't my video card, but the CPU that was my problem, despite the fact that my CPU is benchmarking at around 3.2 P rating.
The consensus was that we all need 10ghz cpu's now......


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&