IMO get the 275 then. If you overclock it/get an overclocked edition, it's faster than the obsolete 280 and gets close to a stock 285 (maybe even faster, not sure), while still costing less than what I paid for my 8800GTS 512 at the time. If DX11 arrives and you change your mind, you might be glad you didn't spend so much on "barely DX10" gear. Or if you want to stick with the GTX, buy another one instead. Introduction of the new cards should bring a pricedrop to the GTX200s.
(The point in waiting for DX11 cards IMO isn't DX11 support, but
full DX10 support (10.1), which uptill now only Radeons have. At least you'll be able to fully enjoy DX10 titles untill DX11 takes off. I think it will be excepted a lot faster than DX10, since noone likes Vista
but everybody loves Win7. And when it does take off, you're ready for it.)
Do you have a monitor capable of high res? SLI performs/scales best on high resolutions. If you do, 2x 8800/9800GT will beat a 285 while costing less than a 275.
Single card still rules on lower res though.
(another thing to consider with SLI: you'll need a C2D, preferably C2Q, CPU at 3GHz or higher to avoid bottleneck there. Also a high native res LCD or good CRT, because SLI barely offers any benefit at lower res)
Feel free to ignore, just throwing some info at you so you know what you're getting into.
