There are three major things to check when you encounter these types of rebooting issues -- two have been mentioned so far.
Power Supply - due to capacitor aging a PSU that was/is fine under light loads slowly degrades under heavy loads. A good rule of thumb is to use a power supply that's rated 30% more than your total system thanks to this, as after two or three years of use that's what it will drop to in capacity. This is even more true with todays high-end gaming cards where the GPU is sucking as much if not more power than the CPU. You can use a tool like:
http://extreme.outervision.com/psucalculatorlite.jsp
To get an idea what you should have for a supply. I HIGHLY suggest plugging in 30% under 'capacitor aging'.
Memory - RAM failures are the leading cause of BSOD's in brand new systems, but it's rarely an issue if the machine is stable under normal loads and only buckles when pushing it -- like gaming. If it's only having issues when gaming, I'd put that at the bottom of the list of things to check.
Airflow - Is your case REALLY providing enough airflow? Depending on the system a lot of machines (Apple's come to mind, don't even TRY to game on a mac pro) wouldn't know proper cooling if it stripped naked, painted itself purple and hopped up on a table to sing "Oh look at what a big cooling fan I am!". Are your top and rear fans pointed to blow out, front/bottom/side to blow in? Remember, in AND out (I'm shocked how often I see machines where EVERY fan is blowing inward!)... when did you last de-fod your fans? You'd be shocked what a can of compressed air can do.
Do you even HAVE proper fans and vents for the level system? I've been seeing a LOT of people stuffing i7's with 200+ watt ATI or nVidia video cards into cases that have no top vents, anemic rear vents, no side intakes, and barely passable front fans. These are components that can raise the ambient temperature of the room they are operating in 8C
or more -- Imagine what's going on inside that little case for temps! Putting them in a case with improper airflow means you're killing them prematurely. If you are worried about the noise of so many fans that's what the larger (180mm+) fans at lower RPM's are for -- either that or you have no business gaming.
It's why I've got my i7 870, GTX 560TI and GTX 260 system (GTX is for dedicated cuda/physx and to drive 2 extra displays) in a TT Element G, with the top and front 200mm fans, rear 180mm fan, and side 230mm fans -- with a TT Frio on the CPU (with arctic silver)... Just as with the power supply I've got a 850 watt TT TR-2 for a system that only adds up to 600 watts.
Could be as simple as your CPU cooling. Are you on a stock cooler with the stupid 'sticky pad', or are you on a real heat sink and fan applied with a decent compound like Artic Silver 5 or Tuniq TX-4? Compound ALONE can be 10C at the CPU. When the compound was applied was it slathered on willy-nilly, or was it applied properly as a micro-thin even coat since the only purpose of compound is to fill the microscopic gaps, NOT to replace actual surface to surface contact! -- the little tiny 3.5 gram containers most compounds ship with should be enough for ten to fifteen CPU's!!! You are NOT FROSTING A CAKE!!! :D
... and that's a big point in terms of stability; excessive heat can compromise the CPU, RAM
and video card!
What are your system temps? Coretemp, speedfan -- handy utils.