The videocard is almost always the bottleneck and overclocking that most often gives you better performance in games.
Yes, OC'ing is rather complicated at first look, however it doesn't take very long before you are familiar with it. There used to be excellent guides for overclocking Athlon 64s, and they're still around, but they seldom apply to the Core 2 Duos. I haven't, as of yet, found any good Core 2 Duo overclocking guides.
The best I can do is recommend you to this guide
about Overclocking in general (not specific to either company). Afterwards, you should know enough to use any guide found through Google to overclock your C2D.
HINT: The warranty is only voided if they know you overclocked. Since there is little chance of you damaging your components (there are failsafe measures in place to prevent damage from things like excessive heat), all you have to do is set the processor to it's default speed before returning it, and that's if you even have any problems with it.