How does the processor affect the performance of a gpu?

Hey I have this question about processors, Is it possible that one could have a very good high end card (radeon HD series for example) and a low end processor (celeron d) and get choppy graphics and slow frame rates in games due to the processor?

Yes. What you are describing is a common phenomenon known as bottlenecking. This occurs when one or more components of a system are not able to feed data at the same rate as other components, which creates a delay. This shows up as less-than-ideal performance. Most new cards upto the level of a 7900GT or so will work quite well with a newer Prescott-based Pentium 4 at 2.4GHz or so. But anything beyond that, such as an 8800 series card or an X1950XTX will deliver poor performance.