With very few games coming out before windows 7, dx11, and AMD dx11 hardware which will likely in single card be more than enough power for any current game, and quite possibly faster than a 295 in newer dx11 games I can't see a single reason to waste cash on a Nvidia card right now.
the 295/285 are already horribly price cards compared to a 260gtx 216 core which you can get for what, £130 anything else in the entire Nvidia range is a joke in terms of value. The 260 has 90% of the 275 performance, 80% of the 285 performance and 60% of a 295 performance all for £130. 2x260's and a 295, without looking inside the computer you'd NEVER be able to tell the difference, your wallet would however, ok so sli = specific mobo's = a pain in the behind.
If it has to be nvidia and you don't have a 2XX series gpu, get a 260 216 core to tide you over till AMD/Nvidia dx11 cards.
Keep in mind the very real possibility that with dx11 games(which WILL come thick and fast keeping in mind all the big developers were supposed to release what were originally dx10.1 games up to 2 years ago before Nvidia had MS hack dx10 to bits, meaning for 2 years they've had experience with dx10.1, had it ready to go, and just waiting for a standard to enable it all properly) all AMD cards for the past couple years will suddenly have Tesselation and several performance boosts available making them even better bang for buck tbh. ALso keep in mind a lot of the delays on dx10 games were basically having to cut down and change dx10 significantly, up till a few months before release they were all designing for a very different DX10, when it was changed they had to radically change their games(with little incentive in a heavily crippled API they'd spent lots of money getting working in the first place). DX10 was set to come out in new games thick and fast before the change with very real performance benefits, only after it was crippled and a tonne of extra work was introduced did DX10 become useless. DX11 simply won't be the same story, its a pretty radical change with lots of very very useful improvements. Infact, its very possible that a whole heck of a lot of current games with DX10 in could be patched up for dx 10.1/11 very quickly as most of those games would have been originally intended for DX10.1 anyway. I mean Assasins creed almost certainly will have its dx10.1 re-enabled, how many other dx10 games are really dx10.1 with a bunch of things disabled?
No matter how much cash you have, unless you game on a 30" screen theres a handful of games a single 260/4870 can't handle, theres even less a 275/4890 overclocked can't handle. Exactly 3 months away from DX11 and probably only 2 months away from the first dx11 hardware, with the normally very dry Summer months ahead for gaming I'd say spending on a high end card at this point is the worst move.
The highest end cards devalue the worst when new tech comes out, so buying the newest card, at this stage is insane, especially with the deals on a 260/4870 right now.
hell, picking up a 2nd hand 260/4870 now from those who are needlessly upgrading offers even better value right now.