Both are pretty important, but if I had to choose which is more important in games, I'd say that video cards give more performance. My brother has a 9800 pro with a stock AMD XP 2500+ (1.83 ghz) which isn't fast at all, and in most game right now he can play them at pretty high settings with very decent framerates.
You could put a 5900xt in 1gighz duron and play source. trust me i know.
But my athlon 64 3000, overclocked from 2gig to 2.4
Cant run source with a geforce 2.
CPU or system performance help you get to new levels and with some games you may need more power or bandwidth to allow your Video card to do better. It also helps a lot with other things with the game other than display.
The Video card is the main hardware when dealing with FPS. It is what actually renders the scene. A good card can get you through those new games but without a decent system and enough RAM you may still fall behind.