I agree that a 5970 is the best single card solution but not the best...
If you want the best; I would say 2 GTX 480's considering it beats the hell out of ATI's 5000 series in most directx 11 games and is a more future proof purchase then the 5970 considering how the GTX 480 handles tessellation and I think more games will be built around this tech. You also have better driver support from Nvidia and physx and cuda built into the 480. ATI has great hardware but still lacks in the driver department.
My last 4 year history with graphic cards have been...
1. 4870x2 paired with a 4870 = tri-fire and it ran great, no complaints
2. 2 5770's crossfired, ripped the heatsinks out and replaced with Arctic Cooling Accelero S1 Rev.2's, I was able to overclock the vcore to 1030 and memory to 1300 and still stay below 68 degrees. Ran better then expected but lacked in directx 11 games such as BFBC 2 with minimum fps drops. and driver support was slow in certain games.
3. 2 GTX 480's running in SLI - RAW performance is all I can say, runs crysis better then the 5970 but kills it in most directx 11 games such as aliens vs predator and Metro 2033 and BFBC2 never drops below 45 fps and avg around 75 fps everthing set to high 32CSAA.
Temps never heat above 78 degrees on full load and idle around 38 - 41 depending on room temp.
I was eyeing the 5970 but guess what, found 2 new GTX 480's for around the same price on craigslist so I would say if you could find a deal like that; go for it.
Otherwise, I would suggest 2 5770's is the best bang for the dollar period, 100 dollars cheaper then a 5870 and out performs it, I compared both.
Not everyone told you that you need a really fast rig to push a 5970 or 2 480's I had to do a complete rebuild from a q9550 to a I7 960 which made World in Conflict jump from 54 fps to over 120 fps, That pretty impressive. I was able to get the 960 for 300 dollars as well.
Look for deals but if you cant wait, go with my suggestion @ 300 US dollars, you will be happy