should give you a pretty good idea of what to expect.
I've seen some benchmarks that give tri-9800GTX the edge in certain environments; but for the x2GX2 you're looking at a ~$300USD more in price for a marginal, if any, real-world performance gain.
It's mostly a driver issue at this point, and in an ideal world where hardware driver's were 100% efficient in all configurations the x2GX2 would be much better. But w/ the new gen cards just around the corner, and the $1200 graphics card market not being a huge profit generator, the current driver iteration is more or less what the most refined they're ever gonna be. Tri-SLi drivers are simply better than their quad counterparts (they've had 2+years of tri8800 experience).
And don't get all knicker-twisted about how well Crysis plays. It's not an example of graphics prowess, but an instance of how much poor coding can design can slog down an otherwise capable machine.