I think it depends on your budget, so how much are you looking to spend for a graphics card and a cpu cause depending on that the members of this forum can find you a great deal and performance for your money.
As for AMD v.s Intel, They are saying that Intel is switching to a new socket and the AMD AM3 socket is going to be around a lot longer so you could upgrade your CPU more so with a AMD v.s a Intel in the future, myself I think that Intel has better performance in games.
I have some questions for you:
Whats your budget?
Are you in the US?
Did you need a motherboard, graphics card and cpu or just the last two?
Are you for sure looking to crossfire or SLi?
since you don't upgrade very often (your current PC is 6 years old), i would go with the i7 for $200. I personally have a x3 435 that is OC to 3.3ghz but I doubt I will keep it for more then two more years. plus the i7 will let your 5850 hit its full potential, the x3 would be better with a 5770 or below.
If you want AA & AF at higher levels, its not the CPU you need to worry about. Its the GPU. It sounds what like you are looking for is more the 5870 than a 5850. Either processor would do fine. But imho prices are so low, if you go AMD, get the X4 955. No sense in getting an X3.
This is my thought as from what your telling me and from what I've read since your not upgrading that often your what the i7 series or the AM3 with T6 cored AMD, because I just saying the bang for your buck on the AMD 6 core is so nice, but as well for the i7 performance is so really nice! If you have the money get an i7.