some games are developed in a way which rely more on GPU and some which rely on CPU.
yes, I agree with the person above as there are several factors which might be running parallel along with the game. You should cross check all the programs and services running behind and make sure your Hard Disk Drive (HDD) is defragmented to make sure you get the optimum performance.
one more thing, as you said that you doubt that it might be a bottleneck for your GPU, then I would like to say that its quite hard to find out which hardware GPU or CPU is a bottleneck for the other. It runs on a global rule, more the power, more the performance
Bottleneck in these cases can only be stated when there is a mix of a low end with a high end configuration. for example, ATi HD 6670 with a Core i7. This is because the frequency at which i7 works is quite faster than of the 6670. The money could be wisely spent on a configuration like 7750 and i5, as this combination would churn out more power.
To stay out of the Bottleneck area, always buy equal level of CPU-GPU configuration.
You can check out www.tomshardware.com for a hierarchy chart which will be very helpful for you.