Yeah, it will be a while before any game is exclusively DX11. XP is still in wide usage so it doesn't make financial sense to make a game that can't run in DX9 and even after that a majority of people would need to have upgraded to a DX11 card before it makes sense to drop DX10 support. You just wont be able use the advanced settings that DX11 allows for in games that use them.
Just to be sure there is no "confusion" for the OP, the GTX295 cannot and will never be able to play exclusively DX11 games; however, it will take a really long time before games are released as DX11 only. Even now, 3 years after the release of DirectX 10, I'm still not aware of any games that works exclusively with Dx10, they are all backward compatible with Dx9.
DX11 brings a number of enhancements, most notably better multi-threading support, that are not dependant on the GPU. That said a DX10 GPU will not be able to render a game that's been designed for DX11 only. As the others have said though it will be some time before that happens, but you can expect developers to pay less attention to the level of detail for DX10 only GPUs as DX11 becomes more popular.
How may times are you going to ask?
First off the HD4890 is more expensive than an HD5770. Comparing 2 higher priced cards to one lower priced card makes no sense.
Second we've already answered you with all the info you need. You don't have to ask over and over, the cards aren't any different now then when you asked before.