SLi > Crossfire
Basically
Quite probably. But when I last dabbled with SLi, the results were less than impressive vs. the cost. I bought a 7900GTX at about £245 when they were good cards to get. Later, a bought the same card again off eBay for £99 from a chap in Scotland.
SLi'd those mofos together and felt smug in my quite potent setup for the time
. Ran the Half-Life 2 : Lost Coast benchtest and got 90fps on one card, 112fps in SLi. My m8 bought a generic, unbranded ATi 3850 for about £70 and it pissed on my setup at 160fps+.
I'm far from saying that either technology is the best, nor is there a benefit in having two cards under the lid. But I just think in real terms - by that - what you actually see going on with your own eyes on the screen - the additional card really doesn't amount to much. The benchmarks may tell you differently and perhaps in one game, an SLi or Crossfire configuration would clearly have a significant edge. But generally across the board, such gains are really few and far between. I'd rather cut back on the fan noise, the power used and allow better air-flow through the case by just using a single, decent card.
D.