I'd say buy 5870 now. You can find them. A little perseverance pays off. I had to fight to get my pair of 5870's, but I finally got them from Zipzoomfly. Newegg can't keep em in stock hardly, but I've been able to find them at Tigerdirect and zipzoomfly.
I didn't want the Diamond branded 5870 after the one I got from Fry's died after 4 days of use. Zipzoomfly had the HIS version, so I got a pair of them. As soon as I hit buy and finished the transaction, I went back and looked at the page and they had gone out of stock. Apparently I got the last pair. The next day though, they had stock again.
When my Diamond card died, I was going to be stuck a few days without a video card while waiting for the HIS cards to get here, so I temporarily picked up a EVGA GTX295 from Fry's in exchange for the dead 5870, paying the difference. For the 3 days while I waited for the HIS cards to get here, I tested the GTX295 up one side and down the other, in exactly the same things I beat the first 5870 up with. Furmark, 3Dmark, Farcry 2, Crysis, DoW2, X3 Terran Conflict, Fallout 3, the whole gamut of stuff I have to throw at it. While in many scenarios, the raw speed was indeed a hair faster then the single 5870, in many others it wasn't. And the default IQ of the GTX295 was just plain UGLY compared to the 5870. And many of the tests I had lined up had Dx10.1 or Dx11 support, which of course the GTX295 doesn't support either of. I was so happy to take the GTX295 back to Fry's when my HIS cards arrived. PhysX means far less to me then non-proprietary standards like DirectX.
I used the store credit from the GTX295 to get a second Torqx SSD and a nice printer :p