What games are you playing and what FPS are you getting?
Remember anything above 60FPS, you will not be able to see the difference since monitors refresh at 60 FPS. You might not need to upgrade at all and just save for a new i5/i7 or AM3 build. The 4870 is still a good card at your resolution.
True, LGA 775 is at the end of the road, but the fact remains that you have an LGA775 setup already and a board that will accept quad core CPUs. The extra cores and cache will certainly help especially as games are getting better at being multi-threaded. If you want to keep your system for a while, go with the quad and 5850. If you plan on getting a new board and CPU within 6 months then go with the 5870 and a better CPU cooler to push your CPU more instead.
tecmo : In modern wrfare2 and dirt 2 I see some hitch here and there.
Megamanx00 : You are right, I don't plan on getting any other update for at least 1-2 years since I'm about to go at a college that cost a good amount of money.
So the q9400 is good enough to hangle multicore games ? Is there a big difference between the q9400 and the q9550 ? 6 vs 12 mb cache ? Or only on paper ?
I think that the 5850 is powerfull enough and will provide me the new dx11 so I'll be good for a good 1-2 years on the gpu. As a small e5300 is filling my needs right now, the q9400 will probably be the best for that 2 years to come without any upgrade !