Worth upgrading?

[Sq334_pegasus]

Hi all,

I already have CoD and while it runs fine I would like to play it at higher settings.
Would it be worth upgrading certain parts of my pc or would it be better to save/wait a bit longer and get an entirely new system?

What I have:

Q6600 (not overclocked so at 2,4GHZ)
4GB DDR2 667
HD5770 Vapor-X
Windows 7 Home Premium 64
400watt PSU


Would looking into overclocking my processor give me much better performance? I use the stock cooler now but could get an aftermarket cooler of course.

Any advice would be most welcome!