Yeh, for that resolution, 2xGTX260 216 Sli or 2x4870 Xfire/4870x2 is the best option.
4850x2 in quadfire looks pretty tempting for $500
Just go for the 2 GTX260 SLI or 4890 crossfire. If you have the need upgrade wait for this summer when they release better cards. Ati's 5800 or nvidia's 300 cards should be very promising.
id go with tipmans suggestion. 4890s crossfired and overclocked to 1ghz(if you get a couple good ones) would make a hell of a machine. i contemplated selling my 295 for this option but the gain would be negligable.
^ Quad fire scales poorly. Tri-Xfire/SLi is the max you should do.
ie. Get 1 4850x2 and Xfire that with a single 4850