I have an similar issue ..
I'm running windows 7 x64, Rampage Extreme, Intell QX9770, 8g pc12800 ram, HD 2870x2 with asus P1 phxsics.
The problem is that I'm getting "small box's of green, red, purple and white" on boot up and even after the computer fully boots to windows.
and it take servral restart to get to run normal..and this is only for a few hrs before I get "display driver has stopped working but has recovered". and the same box's suddenly appear and forces a shut down and or reboot.
I have tried other my other GPU, OS (Vista x64), fan setting, clock downs and still getting the same problems.
I had never had this problem eailer in the year of 2010.
Do remove the cards as Maziar suggested - always start with basics. Remove cards you dont need, unplug drives you dont need to see if that helps. I question why you would try Vista or another GPU - unless it was a 4870x2 also.
The main issue with these cards is either power (lack of) or heat (too much). When graphical issues happen either your current/voltage has fallen or the system cannot get rid of the heat.
I didnt see what kind of 4870x2 you had - was it a reference design? 4870x2's run HOT as Im sure you know.
On the reference card there were 5 leds on the side - if any of those light up you have a power or heat issue. You can google for the information on them.
The fins on the heatsink collect dust like mad, have you checked them out?
Sometimes one has to replace the heatsink compound on those cards....