So, you're running Snow Leopard with a GPU which isn't known to be troublesome.
You're running a beta version of CrossOver. You should be running the actual release version of CrossOver Games 9.
One possibility is that your Mac has not completely locked up (a window server hang or kernel panic). Instead, the game which is running full-screen has locked up.
If that happens, try Command-Option-R. That's a keyboard shortcut telling CrossOver to exit out of full-screen mode. That way, you should have the ability to clear things up. You can use the Task Manager (taskmgr) control panel from the Manage Bottles window. Alternatively, you can use the Quit Bottle and/or Force Quit Bottle buttons on the Bottle tab of that window.
If Command-Option-R doesn't bring back the CrossOver menu bar, the you can try force quitting CrossOver itself with Command-Option-Escape followed by Return.
If neither of those work, then that suggests it really is a full-system lockup. That may be a hardware problem. Your GPU may be heating up. That may be an issue for Apple to investigate.
You might experiment with playing the game in windowed mode and with graphics settings turned down. The windowed mode will let you more clearly distinguish between the game locking up and the whole computer locking up; if the game stops responding, you can try switching to other applications by clicking on them or with Command-Tab.
The lower graphics settings might avoid any potential problem with the GPU overheating.