ATI 4878, 4870 or 4870X2 or 4890 Crossfire

I have already chosen my CM, CPU and Ram.
Asus P6T Deluxe
Intel Core i7 940
2 * 2 GB DDR3.

And I use the Antec 1080 case.

I plan to make a raid 2 * 500 for the system and 1 to 2 * for storage. A prior I would feed on a 850, although I think it's a bit much. But where it gets stuck it to the graphics card. I expect to play recent games with the best quality. It is a pc to make me happy So.
The question : It is worth to taking a 4870X2 ?
Can I take a single in 4870 Then according to my needs, buy another and make a crossfire? I direct or darkens a 4870X2? Knowing that the 4890 comes out, what choice had I?

Why not go on for your triple channel RAM? Then I take a X2, not much more expensive than 2 * 4870 basic, especially as buy a 4870 later, they say and then we do not because there is more at that time.

It depends on your screen if you have a nice 30 "a 4870x2 or 4870x2 4870 (to be comfortable) see a gtx295. After 4890 the cross of the air to poutray good but not enough power available for testing give a real opinion if you choose this option => attends modified versions.

I'll stay with XP, I know, shame on me, by then. Xp manager that does 3 go, I think to go from 6 (ie 3 * 2) in 7. But unnecessary at this time.

The problem not from windows xp or seven but only the 32bit version ... managed to more than 3GB of ram you have to go in 32bits (windows xp 64-bit managed very well over 3GB of ram ....)