Mac Pro 2008 graphics card crashing
It does sound like the graphics card is failing. The garbled display is a symptom of this. 

If you only run Windows, any PC graphics card in your Mac Pro will work fine. 

What's more, the PC Radeon 58xx and 68xx cards work in OS X too, out of the box. 

I would recommend one of Radeon 5850, 5870, 6850 or 6870 if you want Mac compatibility, or any good graphics card within your budget if you only want to game in Windows.