PS or Graphics card causing DTD

My wifes eMachine T2865 is having a problem. I installed a Geforce TI 4200 128MB video card to replace the onboard video. When she played any game that required more than just basic graphics (Links 2003 & Tiger Woods PGA Tour 2004) the PS would become very warm but not extremely hot. Then after just a few minutes the computer would drop to desktop and once or twice it rebooted. The PS cools off very quickly when it drops to desktop. This sounded to me as though the PS (150 Watts) was simply too little. I replaced it with a 300 Watt. The same thing happened every time she would play either of those games. I then removed the TI 4200 thing it was a possible problem and reverted back to the onboard nVidia graphics. The PS still gets very warm when those two games are running but they both will run for a longer time before they drop to desktop. I have checked MSCONFIG and taken everything out of startup that is not necessary. I have also gone thru SERVICES and done the same thing. eMachines tech support has been minimal and of no help thus far. All the fans are working and it is not reporting any CPU overheating. I am baffled by this one.
Specs:
Athlon 2.8 (I believe, I'm at work so I can't check it)
512MB
nVidia onboard graphics (something 440 I think)
Windows XP Pro
AC '97 sound