I have an Asus A8R-MVP Deluxe with an x1900xtx, X2 4800+, 2 gigs RAM, and I'm considering various upgrades.

The easiest upgrade would be adding another vid card. Will I see a performance increase (better frames when using 16x anisotropic in The Witcher for example)?

Does it have to be the exact same vid card for crossfire?

And will it be much noisier?