GTX 275 SLi = GTX 295. The HD 5870 trades blows with the 295 and it's cheaper, uses less power, only requires and single PCI-E slot, and you'll be without all the disadvantages of a multi-GPU setup.
I say use the SLI. I think an SLI'd 275 would slightly out perform the 5870, but would outperform it in games with SLI support. Therefore it would rape. Also, 5870 just came out so it's bound to have bugs and what not. Only downside with 275's is no DX11 support. Maybe just wait and see what nvidia comes out with and if the prices drop.