i've never played CSS on a nvidia card, i haven't used nVidia since geforce 4...i don't have any problems with ATi so far...i modify color settings in the CCC advanced profile and i turn the brightness up so i can see the noobs hanging out under the trains...
about nvidia's DX11 hardware, it'll be about febuary before you'll see anything, they don't even really have anything to show at CES as far as i know...and even then, the high end will launch first, so you may have to wait till march or even may for a mainstream product for nVidia's DX11 lineup...
and also depending on what you're playing the gts250 even being a rebrand, is still a G92, solid chip, even the gtx260 and up is just a die shrink of G92...with more transistors, shader units and whatever else, same design/technology