HD 4850 X2 vs GTX 280 vs GTX 275

Don't get GTX280.

GTX275 performs almost at its level for far cheaper.
A 4890 performs at that level as well.

Lets just say that... hypothetically

GTX280 = $270
GTX275 = $250
HD4850X2= $260

Do not compare stream processors of ATI and Nvidia, it is not the count that matters. VRAM realy doesn't matter above 1GB (NO game will benefit of more than 1GB VRAM,unless you play ultra high resolutions with AA/AF, but then it does NOT matter because you'll be GPU bounded anyway)
Pick the games you wanna play, pick the resolution and google some benches of the cards and you'll see which one is better for the games you picked and the resolution you want.
I'll go for HD4850x2 it is definitely faster than the nvidias on most games, unles you want to play some crapy stuff that doesn't support crossfire. If so - then go for HD4890 which is cheaper than both nvidias and it's highly clockable. Clocked to 950+ Mhz will be faster than clocked GTXes (note: 99% of HD4890s on the market can hit 990+Mhz)

get the GTX 260 core 216 models, they can be overclocked to match the GTX 280 and can be found below the $200 range. You can't go wrong with it. Use EVGA precision tuner for OCing it, save yourself some money instead of buying the factory-OCed versions.

I understand there are 4 DVI, but you can't span one desktop over all 4, you have to have 2 separate ones. Meaning that if you play a video game, like an FPS, you can't have 2 side and one main screen. You can have two put together.... but that's nothing desirable.

google it you lazy ....

http://www.google.com/search?clien [...] gle+Search