With a single 5770 I doubt the Q9550 is creating a bottleneck for you. You can certainly try OverClocking the CPU above the 2.8Ghz stock speed and see if your benchmark results increase (for example with 3dMark06 or similar). If your GPU scores increase just because of a CPU OverClock then it was holding you back.
For general gaming purposes, the only reason that CPU would hold you back with a single 5770 would be in cases where a particular game is very CPU demanding.