Viper's Lair posted an interesting article on their site about bottlenecks :
I'm writing this because I still get an awful lot of emails asking me to explain how a video card or CPU can be a bottleneck in a PC setup. As I've said in several reviews and articles, a video card bottleneck occurs when the CPU is providing more information than a video card can handle. A CPU bottleneck happens when the video card doesn't get enough info from the CPU.
Raedon 9700 PRO
To understand how video cards work, almost all modern video cards need a decent CPU to go with it. If the video card is too powerful (for example, a GeForce 4 ti4600), and the CPU is too slow (a Pentium 2 266), the video card is wasted because the CPU can't deliver info as fast as a video card can take it. We're not saying that the video card won't be an improvement over an ATI 16MB Rage, but don't expect 300fps, that's for sure.
GeForce 4 MX440
Like you see on the benchmarks above ; the results for a Radeon 9700 Pro and a GeForce 4 MX440 are almost the same on the P3 500MHz.