Hm,I wouldn't consider any upgrading atm. GTX280 is a very nice performer @ 1920x1200, but if you still do insist on selling the beast, go for the 6870 model, I somehow don't really think the 6850 is much of an upgrade over the GTX280.
Maybe waiting a few months sounds like the best idea, just to see how the price / performance ratio will cycle over the period, and the appearance of new drivers for 6XXX series..
