Right now I have a Pentium Dual Core E2160 oc'd from 1.8ghz to 2.7ghz a radeon HD 4850 and an Asus P5K Pro mobo. I'm thinking about upgrading but don't know whether to upgrade my CPU to an Q6600 or even an E8400, or just add another 4850 and go crossfire. Any thought about which would be the better choice?
You have a reasonably balanced system now. If it is for gaming, then you should probably upgrade both cpu and vga. Upgrading the vga part will probably be the more effective path. I think the E8400 would be a better upgrade than the Q6600, unless your game is FSX or supreme commander which are multi core optimized.