Yeah, that's a pretty big mismatch of parts and speed. If you can afford it, do the whole new system. The GTX560 is way, way more video card than the AMD rig can push, and the 7800 is so dated it would not even fall into the bottom-end category for a modern gaming GPU.
Well, it has served me well since 2005, and has played every game at max until Crysis came out. Then I replaced it with a horrible HP laptop, which basically makes the same amount of noise as a small fighter jet engine and delivers equal amount of exhaust heat..
So generally a bad idea.. but if had to be done, it I should go with the cpu + mobo combination.
Just wondering - how powerfull is the onboard graphics of the i5/Z68? Can I watch movies / play casual games?