Help! Hard crash when gaming

[Zimbop]

When playing Raven Shield and also Second Life, I get frequent (within 5 minutes to sometimes half an hour) hard crashes that start with the top half of the screen going dark (like a visor dropping down!). This is usually accompanied with some sort of loop going round and round in the audio (often a machine gun or something). Sometimes a power switch icon comes up with wording to the effect that I need to restart my Mac.

I'm using MacOSX 10.5.5 on a MacPro 2 x Quad-Core Intel Xeon 2.8Ghz with 6GB of RAM.

I've no idea how to start to diagnose this one. I can't think that I've installed anything very different recently that coincides with the problem.

Any help appreciated.