8800GT needs upgraded, but which GFX card?

I'm presently running my E8400 Oc'd 4Ghz system with a BFG 512Mb 8800GT OC2. The system runs most games (stalker, left for dead, crysis, supcom etc) at near if not max gfx settings (minus/low AA) at 1680x1050 res on my 22".

With up and coming titles like Empire total war I know that with the 8800GT my system will have smoke coming out its rear! Will need to be upgraded to get the most of the gen games. Question is which card to go for; GTX285 (in place of the GTX280), GTX260 or the 4870. (4870X2 and GTX295 out of my price and power range (550W PSU)).

Any suggestions or pointers would be appreciated.

Cheers

Andy