Just bought an old graphics card and when playing Guild Wars it keeps restarting?

Ok, today i bought an old ATI Radeon 9550 SE AGP slot and 512mg RAM for my PC because i wanted better quality while playing Guild wars. looks awsome, but every now and then when im playing, sometimes it will just restart my computer without warning. its happened 3 times. whats the problem? im in a 28 degrees celcius room, is it too hot? is something wrong with my fans? Guild wars suggests some memory test which i cant do, its annoying and i need help. thanks.