Any linux experiences with two PCI-I gfx cards?

At the moment I have a Radeon 5450 and a Radeon 9250 PCI installed in my X1000. It works fine in OS4.1 but I can not get 3D acceleration in any linux distribution except the officiel Ubuntu Remix 12.04 from A-EON.

I have been offered a cheap used Radeon 6870 and I was wondering if anybody have any experience with two PCI-E gfxcards and Linux? I know it runs in OS4.1 (no native 3D) but how does it work in Linux?

Btw, I have a 650 Watt powersupply that should be enough for the 5450 and the 6870, or?