Get a single 4890 for now, it should play most things on high fine, and if you want a second one later one the 58xx series will cause the 4890 to drop in price.
With your budget you should go with an i7 system. You can still use crossfire on the x58 boards. Getting an i7 and a mid level x58 board won't even increase your budget.
Once you pass above 60 FPS it doesnt matter as your monitor cannot display it unless you paid alot for one of the 120Hz ones. If you look at this review you will see that everything except crysis was getting 60 or more at max settings with no AA
http://www.tomshardware.com/review [...] ,2380.html A GTX295 is 2 GTX275s in SLI, and you can get a GTX275 clocked higher than a GTX285 easily for significantly less. Unless you feel the need to run everything at max settings with AA enabled you wont make use of the second card atleast not for a while, by which time spending 200 on a modern card will blow 2 old cards in SLI or CF away.