My $0.02
I had an AthlonXP 2400+ (266Mhz bus) with a Radeon 9800Pro and 1GB PC2700 and I was definately being CPU limited. A friend of mine had a 9700Pro on an Athlon64 3200+ and he outscored me in Aquamark3, 3DMark2001, and 3DMark03 (by a decent bit). It was only in 3dMark05 where HE became seriously GPU limited that I pulled ahead. I was getting about 2400 and he was barely breaking 2000.
Now: before people start yelling at me. My computer was definately faster with the 9800Pro than with a lesser card. We switched cards and I did worse in all of the benchmarks. This meant that YES the GPU still scaled to the processor, but wasn't necessarily running to its full potential. I'm sure that if I put an x800, 6800, or 7800 class GPU in my old system that it would be even faster, though still held back by the CPU to some degree. Since I don't have any of these cards I couldn't tell you at what point a faster GPU would stop helping.
Sorry I couldn't be more helpful.
-mcg