Am I entitled to a change of opinion?
If the OP is still interested, if it is actually a sure thing to be able to get the 5870, or you have a reasonable card to use right after you sell that 295 and wait perhaps a week or two or more for a 5870, then sell the thing will the price is high and order a 5870.
I believe the 5870, even discounting DX11 for the moment, will be of greater value. You won't need water cooling, it's a lot less power hungry at idle and at load compared to that 295.
As for DX11, you're covered no matter which way the wind ends up blowing:
Scenario 1, DX11 doesn't become mainstream fast enough: You still have a card that is powerful for any game out there. Maybe with the sloppily-coded Crysis as the only exception, the 5870 will smoke anything you throw at it. And, even if only a handful of games get to use DX11, at least you do get to see it for those games.
Scenario 2, DX11 is mainstream faster than the naysayers predict: You won't have to kick yourself for not grabbing a 5870 when you could have had it for cheap (that's the scenario right, you got a 295 for a song, sell it for a profit so as to get a 5870 for cheap?)
Upon re-reading the thread and re-evaluating the possibilities, I have to say I was wrong (heh, wouldn't be the first time)
OP, if you are still interested in this thread, if I were in your shoes I would sell that 295 while it's still a hot item, get a nifty profit, and jump on a 5870. It's a no-lose scenario, practically.