Finding occasional crashes while playing games:

a) Game freezes and a reboot is required

b) Monitor goes to standby, computer reboots but the monitor doesn't
come out of standby. Reboot is required.

Also, when I'm just on the windows desktop the screen sometimes goes
blank momentarily (which normally happens when i switch resolutions) and
I hear the two-note sound that Windows normally does when a peripheral
is plugged into it e.g. usb hub etc.

My system config is: Asus A8N SLI Deluxe motherboard with two 256MB
Inno3D Geforce 6800GT cards, AMD64 XP4000+, 1GB RAM, 2 x 200GB SATA Hard
Disks, HEC 550W PSU, Windows XP.

Any ideas?