The overall system is important. Having said that, the i7 920 is still very capable, and would not hold back any GPU to any noticeable degree. Sure, if you are benchmarking and comparing numbers, you'll see a difference in charts. However, assuming the rest of the system is up to snuff, you won't really notice sitting right in front of it and actually using it.
Shooting for an overclock to 4Ghz is a good idea as Golfis mentioned.