Just get a wolfdale and overclock the living lights out of it. Your board makes it all the more easier. Right from the E5xxx all the wolfdale are heavily overclockable, I yet have to hear about a "bad chip" from any batch. You would get all the juice you need to use that HD5850 at its full potential.
As of right now, the whole idea of selecting a platform can be a tricky decision. The AMD X6s, 140W TDPs, the intel 1366, 1156 and the speculated 1155 there is really no telling which board would be the best buy. Save the upgrade for a year down the road. Your existing hardware still has a lot of fight left in it(understatement).
PS : About the Q8400, it just has a low multiplier, it still overclocks quite decently. With little effort you can reach something like 450 FSB, that would be about 3.6 ghz which is plenty for a quad core. Its reputation is due to the fact that benchers find it less than useless cause of the low multipler and relatively low cache.