I would not go for SLI or crossfire.
I would aim as high as possible and when you feel the need to add another one you just sell this one and buy a new better mono gpu.
SLI-CFX = problem down the road.
I would get 6950-6970 or 570-580.
Concidering your budget, I would aim for GTX 570 Twin Frorz 2.
If you want a dual-card configuration and you can spend a bit more ($500?), I'd recommend 2 GTX 560s in SLI. This will be one heck of a setup.
I've had both crossfire and SLI on the same system (see signature) and based on my experience, there is better driver support for SLI than there is for crossfire. I had all the common crossfire issues (occasional flickering video in BFBC2 and MOH, gpu load staying at 100% after gaming sessions, HDMI/DP audio drivers not installing). With crossfire, every month brought new drivers and new hope of driver issue fixes. With the exception of the occasional flicker being removed from NBA2K11, every month the fixes did not arrive or one issue would be resolved while a new issue would emerge. I switched to an SLI setup and things worked exactly as advertised.
To be fair, all my single card AMD setups never have driver issues.
That being said, I would actually recommend you go with a single more powerful card now and then just pick up another when the prices come down in six months. You'll give up some FPS now but two better cards later are better than two lesser cards later. My suggestion is go for a single GTX 570 or 580 and pick up another one when you can.
Additionally, I'd recommend an 850W or above power supply with around 60A on the 12V rail minimum to accommodate the inefficiencies of aging. Granted my processor uses more power than the 2500k, but my system, measured with an appliance load tester commonly reaches the 750W range during gaming sessions.
Either way, though, whether you go with dual 560s or a single 570 or 580, that will be a great setup.
I would go for a Radeon HD 6950 HD, flash the bios to 6970 specs (you can find a tutorial on youtube) which automatically saves you like 100$. Then later on buy another and do exactly the same, if I remember correctly both cards should come out to be about $540 on Newegg.
reference tend to be noisy tho.
My 5850 over 35% is a jet, no jokes.
You can't compare the "usual" reference cards to the new vapor chambers on the 500 series. They are very quiet, and most custom coolers come in right around the same Db level. The benefit of the custom cooler is lower temperatures, for the GPU, with the tradeoff of higher case temperatures.
I got to agree that the vapor chamber may be quiter, but if you plan on overclocking you can't go with reference, you'll have to crank the fan up and it will be audible.
I heard DCII and TFII can go past 60% fan speed and stay quiet.