How to tell if you have a cpu bottleneck?

Hi, I haven't been getting the performance I want out of my system, so before I drop about $300 on a new GPU I wanted to rule out the possibilty of a CPU bottleneck. Whats the best way to tell?

Specs:

Dual 5770s Xfire

AMD Anthlon IIx4 640 3.0ghz

8gb 1600mhz RAM

Windows 7 64bit

700W PSU

Looks fine to me. Run benchmarking software and they should give you readouts for each area of your PC and you can then spot the lowest performing one. For a much more basic way of showing bottlenecks you can use Windows Experience Index by going to Control Panel > System and Security > System, click the Windows Experience Index and run the assessment, it's not as indepth as most programs but is quicker and is already installed and to be honest, isn't too bad.

Typically whats a good Windows Experience Index number for a GPU these days?

You won't see a CPU bottleneck unless it is the older athlons and phenoms (not II) or C2D

In games I thought that if you replace your GPU with a significantly better one and it yields no higher performance, you have a CPU bottleneck.

Depends on how CPU intensive the game is as well. As others said, benchmark software is the better alternative to swapping GPUs to test that.

Typically whats a good Windows Experience Index number for a GPU these days?

yomonkey