I currently own one HIS HD Radeon 5850 1GB iCooler V. I am planning to buy a Sapphire HD 5850 to crossfire.
1) Any website that compares the performance of both setups (single vs double)?
2) Any suggestions you may have?
3) Power supply requirements? I have a Corsair 650W and my major system specs are Intel i7-860, Asus P7P55D, 8GB OCZ RAM, 1TB hard disk, X-Fi sound blaster Fatal1ty Professional Series, and that's about it.
4) Any equivalent card in the market with 2 x 5850 in crossfire, so I can get a fair idea?
What resolution will you be playing at? Unless you're playing @ 1920x1080 (or higher really) there's probably not much use for Crossfire with the ATI 5850 to be honest. Unless you specifically want to play Metro 2033 with DX11 mode.
At 1920x1080 you could see some benefit in newer game titles, or DX11 based titles if you like to max your graphic settings. Typically though, most game titles in DX9/10 mode are going to run very well at that resolution with even a single ATI 5850.
I only bring up Metro 2033 because it's sort of today's "Crysis" in that it's very, very demanding. It's one example of a game that really can play better with a Crossfire setup.
Assumign you're playing new demanding games with all the settings cranked up, Crossfire with an ATI 5850 could have its place. And your system is certainly capable of handling a pair of 5850's.
Just remember, if your monitor has a max refresh rate of 60hz, you can only use 60FPS. Anything above that is for gloating/bragging only.