probably one of the three
1) the driver , get the latest offical driver or get an older if your alredy using the latest
2) a game issue, check for updates
3) your psu dont have enough juice , get sandra http://www.download.com/3000-2086-10018691.html
goto "mainboard information" scroll down to the middle there youll see the voltages under "voltage sensors" type em down and ill tell you if their good or not
My guess is that the psu isn't stable so it restarts the system. It could also be thta the cpu is overclocked too hihg also and that the system can't run stable. The same thing happens to my friend. He has a geforce fx5700LE with a p4 3.0ghz sokcet 478 and 3 gigs of pc3200 ram. When ever he plays counterstrike source he can run ti ok but as soon as he hold alt +tab to get back to windows his comp restarts too. He has an antec 450 watt too so his psu isn't the issue. First do you o/c your cpu? What kind fo mobo do you have? What kind of psu do you have?
so you are saying that with your old vid card you can play source and not bf2, and with the 6200 you can play bf2 and not source? that doesnt sound like a ram isue to me, though im not very good with troubleshootin hardware.. maybe drivers?