Shake/FCP, Video Card Compatibility

Hello,
I've been using Shake for my film. But I've run into snowballing display issues, and now the entire system locks up after starting up.

I took it to a mac repair shop that trouble-shooted the issue: a bad video card. Since I need a new card, I'd like to get a faster one (current is at 256 MB RAM).

The shop suggested a Radeon HD 3870, 512 MB RAM card, as it would be compatible with my system. But since Shake is discontinued, I don't know how to check if the suggested card will be compatible with Shake 4.1 (and FCP 6).

Has anyone used this card with Shake or FCP 6? I'm especially concerned that the newer HD card will make Shake unstable, and may not work with FCP 6, which I don't think has HD compatibility?

(I have a Mac Pro, OS X (10.5.8), with a 2x2.8 GHz Quad-Core Intel Xeon processor.)

If you have insight, I'd greatly appreciate it!
Thanks!
Therese

Thanks, Simon! I've heard the same thing from a few others. When I get my mac back, I'll know for certain.
Therese