CPU Upgrade worth it?

Hi everyone,

I'd like your opinion about this...i have a pc which is used mostly for gaming (but not really "heavy" gaming-i guess)...indicative games that i prefer are say Pro 2011, Fifa11, WRC, Dirt2, Assassin's Creed etc...

My current config is:

C2D E6400 2.13 OC'ed to 2.84Ghz
4GB Kingston DDR3
ATI HD 5770
Gigabyte G41M-Combo

The system performs quite well in general (with a few, not very noticeable sacrifices in AA, AF and other settings it reaches close to (or exactly) 60fps on my 1920x1080 60hz monitor (VSync enabled) in some games, while in others of course it can hardly exceed 25-30fps (i.e. AC-Brotherhood) - still is tolerable though
The thing is that even in games in which 60fps is reached, i still get slight "hitching" every now and then, which is a bit annoying...I guess that's when the CPU goes 100% usage (usually works at an average of 85-95% load when gaming)

The question is, is it worth paying say 70-80€ (~100-115$) for a mere s775 CPU upgrade (i.e a a used C2D e8500 or a Quad Core)...if so, should i go for 2 or 4 cores? would i see any any worthwhile improvement in FPS and "smoothness" of play?

Or should i just stick to my current config and come back when i have at least 300€ to spend for mobo+cpu (+GPU maybe)!??  

Any thoughts would be really appreciated!

Thanks in advance