As someone with CF 5850s, it's totally worth it (playing 1920x1080).
I went from high settings to being able to max out any game so far (well the only exception being Metro 2033, all settings maxed except AAA instead of 4xMSAA but most cards can't handle it with DoF).
As for games supporting it, basically all games released in the last few years support CF. It's actually more of whether AMD supports the game, because you need to install Crossfire App Profiles, the most recent one added Dragon Age 2 and Crysis 2 profiles. Basically as soon as new AAA titles come out on PC they release a new CAP.
750W is more than enough. I'm using a Corsair 750TX with everything overclocked plus a GT240 for PhysX and a sound card and 2 HDD in RAID0 and an SSD and my case has 6 fans on top of the 4 combined fans for gpus and cpu... according to all the PSU calculators I need 708W with the safety factor and all that jazz (cap ageing and such). So yeah... 750W is fine. 5850s are actually extremely efficient cards.