A lot of people want to know what part they should upgrade to improve PC gaming performance. It usually boils down to getting a faster/more core CPU or faster GPU.

Most current PC games today are console ports. These are more dependent on graphics power. Therefore, it makes more sense to put the money into your graphics card. Get the best graphics card you can afford.

There is a point where bottlenecks can develop but most recent Core 2 or AMD Phenom class processors will handle today's games just fine.

Today's games also value higher CPU clock speed over more cores.

If you're considering a multi GPU setup, don't. Using two GPUs you'd think would give you double the performance. However, realistically, you can only get a 50% boost from this in an ideal world. It's not worth the expense. Get the best single card you can afford.

Image from AMD via The Gadget Blog