I recently (3 months ago) upgraded from an 8800gt to a GTX 275, and I'd say what archie said. ATI is pretty much where it's at now. Nvidia doesn't even have a DX11 card now, and unlike DX10, DX11 seems worthwhile.
And just for the sake of things, running the same games after installing the GTX 275 had some double or more in quality/frame rate. It's a big boost in power, but if I held out a bit more, I would have upgraded to one of the ATI cards.