Q9450 is 2.66 GHz, may be newer/faster than OP's proc. It's not just CPU speed, it's the entire architecture around it. E.g. your 9450 has a bus speed of 1333MHz, the 6000 series is lower than that. It's these little things that can often make a huge difference. If it's not that, might even be file fragmentation or programs running in the background or driver versions, etc.

Genky: you're right, but I'd still recommend higher GPU's than nVidia 8 series and ATi HD3000 series.