In my Opinion, the 5870 is the way to go, or perhaps the 5890 when it arrives, it's a true single-card solution that can handle anything within reasoning. I believe the Fermi cards will be overpriced and run hot and not be too much better in performance. 5xxx FTW right now. Maybe i'm wrong, maybe Fermi will be a beast and be cheap, but then again, when was the last time Nvidia & the word cheap were in the same sentence?
Patience is the key word here. the GTX260 will do pretty much everything you need to do at 1440x900 for a while (decent, but not really high res, IMO) Wait till the new line of nVidia GPU's and then decide whats best. DX11 will not really be that much of a useful feature for a bit yet since there are very few games with it yet and most of those run better and look almost as good in DX9/10 mode. I'm gonna guess the ATI 5000 series is gonna be a better buy since they'll be able to drop price, and NVidia is known for high prices, but who knows- maybe they'll realize they need to actually compete for once.
What's the fascination with numbers ? Those that fixated on DX10 were in the same frenzy you are now and to quote old Willie, it was "Much ado about nuthin". DX11 is only relevant when there's something out there that does something with it. We haven't seen that yet. Given the 2-3 year development cycle of new games, we are not likely to get excited about many DX11 games till late summer or XMas 2011. So, your card won't become obsolete until that time frame.
The 260, according to THG's roundup review, provides "Great 1920x1200 performance in most games".....what's changed ? The 260 and the 4870 outperform the 5770 by about 10%.....but the 5770 can play DX11 games you say......er, no it can't.....at least not at 1920 x 1200. Take a look at the Dirt2 article on THG's home page. Even with the minor implementation of DX11 which THG describes as "almost impossible to notice".
The DirectX 9 code path is so slick and beautiful that it's almost impossible to notice the DirectX 11 enhancements while playing. I consider these enhancements a nice subtle bonus if the hardware is there to enjoy it, but the game is practically just as stunning when running in DirectX 9 mode.
There you have it folks. And while this video is compressed and admittedly does not demonstrate the clarity and quality that you would see on screen, I can say with 100% confidence that in DiRT 2, the DirectX 11 code path offers no reasonable improvement in quality that would affect your enjoyment of the title while playing it.
So for an imperceptable affect on visual fidelity, the effect on performance is described as "colossal"
"Enabling DirectX 11 has a massive impact on performance. The Radeon HD 5750 and 5770 can easily handle the high 2560x1600 resolution in DirectX 9 mode, but in DirectX 11 mode things become very choppy."
So, I wouldn't worry about it. Your "obsolete" 260 will play Dirt2 a hell of a lot better under DX9 then the 5770 will under DX11. With the difference in the game experience as a result of DX11 improvements being virtually imperceptible, what are you going to miss ?
I expect once games like Alien versus Predator hit the street, we will start to see some of what DX11's capabilities are. We won't start to see all of what DX11 has to offer till late summer or XMas 2011. The entry point to DX11, at least at 1920 x 1200 / 1080 is the 58xx series, if that's a budget breaker, save ya money until June, or Xmas time 2010....or 2011 and see how DX11 improves while watching prices drop . Otherwise, keep your resolution down to something the 57xx can handle. It's a foregone conclusion that what you but then will have more performance per dollar than anything you can buy today.....that's the unbreakable rule of PC tech....what comes next is always faster AND cheaper.