Help with first build for Gaming at only $900

Hi guys, hoping someone could help me out. I am going to build my first machine. My budget is only $900. I intend to use it for gaming/video editing/CAD/Scientific and Mathematical Simulations. I shortlisted a few parts as given below:

AMD Phenom II X6 1090T Black Edition 125W
MSI 990FXA-GD65 motherboard
4GB Corsair 1600MHz RAM
500GB Seagate Barracuda HDD
Samsung ODD
Seasonic 750W power supply
Cooler Master Elite 310 case
Sapphire HD 6790 Graphics card crossfire enabled
16" AOC monitor
KBD + mouse
800VA UPS

I am looking forward to upgrade this rig later.
Will this rig be able to perform all that is mentioned above?
Is 4GB RAM okay or should I take 8GB?
Will the Sapphire card fit in the above mentioned case?
Is 750W supply overkill for such a system of should I take something lower (I plan to crossfire this card with another 6850/6870 later)? Any suggestions?
The reason why I went for an AMD machine is its low price and it can be upgraded for the next 5-6 years. The mobo supports AMD FX Bulldozer core processors, 1866MHz RAM, SLI and Crossfire. Intel CPUs are highly segregated and they have a lot of sockets to math their products. This ensures that upgradation would mean changing the Mobo and CPU, which costs a hell lot. So please do not suggest alternatives with Intel.

Any input would be highly appreciated..... Thnx from the start....

ram is dirt cheap so why not go for 8 gigs? the card will fit with no issues, the PSU is good for after you upgrade to X-Fire. I'm not trying to turn you into an intel guy or anything but just FYI, if you buy an socket 1155 intel motherboard you will be able to upgrade to thier upcomming Ivy Bridge processors without upgrading your motherboard. and while this system will work fine for all the task you wish to do, a Core i5 would do the same task faster. but to get into an intel set-up you would need to atleast drop your UPS for now, to stay in the $900 range. Intel has made a real mess of things having so many sockets on the market. but the good news is that 1366 and 1156 have been discontinued. they are moving forward on the socket 1155.

Thnx. I'll keep it in mind.