Intel boards support both Nvidia and AMD/ATI cards in SLI/Crossfire configuration. Only AMD motherboards are limited to only allow Crossfire configurations.
As for which to go with, the new Intel Sandy Bridge chips run circles around AMDs flagship 6 core processor. Not only that but they have been reported to easily be able to over clock anywhere in the 4.5-5ghz range. That's pretty impressive and will definitely help on those programs that still don't support multiple threaded applications. Even if you're only gaming the over clockability as well as Crossfire and SLI support should be a good reason to go for Intel.
If you are just looking for a purely gaming system get the i7 2500k. The only difference between that and the 2600k is it doesn't support hyper threading and its base clock is 3.3ghz as opposed to the 2600k's 3.4ghz clock rate - and it's $100 cheaper at $225
If you want to wait AMD is about to release their newest chip architecture later on this year, but if not I wouldn't buy their old line because Intel's perform much, much better. Also, keep in mind that these new Intel chips are considered "mainstream", while their "enthusiast" chips will be released towards the end of this year or early next. If these mainstream chips can perform this good, I wonder what their enthusiast chips will be like?
Here are links to the above named processors as well as a benchmark review of these chips as well as some of AMD's best:
Intel i7 2600k
Intel i7 2500k