3 Monitors, 2 Video Cards - Upgrade advice please

Hi all, Im hoping some of you folks can advise me on my setup.

Right now I have three monitors and two video cards. I have two 19' monitors on the sides, and one 20-something monitor in the middle. I run these monitors off of two video cards, a Radeon HD 4830 which runs the two 19's and a Radeon HD 4850 which runs the large monitor.

How I use my setup when gaming is I play my game on the one large monitor and then have a show/movie/music on one 19' and email/stock/newsfeeds on the other. I want to continue with this setup.

My question is about getting better performance. Can I use crossfire and run my three monitors the way I have described? When I want to upgrade, can I use a better card that needs different drivers? Or must my two cards use the same driver? If I buy two new cards, can I run them in crossfire/sli in the fashion I have described?

Thanks for any thoughts or advice you may have!