Intel 775 Motherboard+6.0GHz Intel Dual Core crashing problem

Dear sirs,

I'm building my first system with a Biostar socket Intel 775 Motherboard with 6.0GHz Intel Dual Core, 2GB DDR and 512MB on-board GeForce7050. I'm running the system with an IDE 20 GB hard drive (can't afford bigger right now!) and an LG DVDR, both of which are on the same cable (HD master, DVDR slave). I also have one side and one rear fan on my tower case, as well as a heat sink and fan sitting on top of the processor. I'm driving the system from a 450W power source.

In addition I suspected that the onboard graphics wouldn't be particularly capable due to the system RAM share, so have also fitted a Gainward Bliss GeForce 7900 with 512MB RAM onboard. Soundwise, there is an onboard six channel soundcard that seems OK and I also have fitted but disabled an old Soundblaster Live card, specifically for use as a Microsoft Sidewinder-compatible gameport. Finally, my OS is XP home edition service pack 2 and device manager keeps telling me that all hardware/software is running as it should.

Alas, this is not the case! Whenever I play a game requiring serious 3-d graphics, or even watch a movie, the system crashes and reboots after a period of minutes, and upon restart I receive a 'serious system error'. I've a feeling I'm missing something very basic - as mentioned above, this is my first full PC build - so I would be most grateful to receive advice from any of the experts out there.

Thank you.