I have a two part problem and would welcome any advice esp. from users with HP Pavilion a!600n

I have an HP Pavilion a1600n--power supply is nominally 300w, but I think actual is perhaps 280w. I disabled the 6150 LE integrated graphics card and installed a BFG nvidia 6600GT. Lot of problems since when trying to run not-very-demanding games such as Sid Meier's Railroads!--chief problem is flashing or off-color pixels all over the screen. A couple of games that I tried to run e.g. Settlers actually crashed the system. Could this be a power supply problem?

I just bought a Termaltake 500w Purepower 2.0 psu, but the Radio Shack guy said the HP connectors between the mob and the power supply are proprietory & he didn't think this psu would be compatible. PSU box says it has a 24-pin main power connector and an ATX 12v 4 pin connector. From the a1600n specs, it looks like the mob also has this configuration. As I haven't opened the psu box yet, can anyone give me a heads up on whether this new psu will replace my existing one.

Thanks,

Vermont Grandma