1) Also I don't get any video signal from Onboard VGA.
You, also, stated that you could still here the game playing and your friends on ventrillo.
So, that leads me to believe the video (hardware and software) is not crashing the system.
Does not sound like the card, IMO...
More like one of these:
1) Bad driver and\or OS install
2) Bad cable
3) Monitor going
4) Monitor setting (like sleep setting)
5) OS power setting to turn off monitor
6) Bad registry setting
7) Incorrect or faulty settings in the bios
8) mouse and\or keyboard not set to wake the device
IMO, I would be leaning to the power settings in the OS, motherboard, and monitor.
Sounds as if it is going to sleep and the keyboard and\or mouse are not waking it.
Try running this command from an adminstrative command prompt: powercfg -energy
Powercfg command line options info
This will run for about 60 seconds and put an HTML log in your USER\USERName directory.
Some of it says errors and warning, but they are not, if the settings are the way you set it.
Look at to see if something is off (not right).
Just my 2 cents.