Graphics card blue screen

Hey, I bought and installed a Nvidia Geforce GTS 250. Once installed, I turned on the computer and everything seemed to be fine until I played a game and the computer shut down. There wasn't a blue screen or anything, but I realized my computer's power supply wasn't good enough. I stopped playing games that had high graphics, but then my computer began to bluescreen almost every time I played any game. For instance, it would blue screen on the starting screen of Dominions 3, as well as when I was playing Haven and Hearth (A java based game with sub-standard graphics). Thinking that this was all related to my power supply, I eventually bought a new one. I can now play high graphics games without my computer blacking out and shutting down, but it still bluescreens very often whenever I play any game. Any help would be greatly appreciated.

which PSU do you have?and what are the other specs?

My new PSU is a 650 watt Antec ATX12V v2.2., and here are the other specs of my computer:
Processor: AMD Athlon 64 Dual Core Processor 5000+ 2.60 GHz
Installed Memory: 2.00 GB
System type: 32 bit operating system
And I'm running on Windows 7

EDIT: I'm not too sure which graphics card I was using before this, I think it was something like a Nvidia 6900, or something similar to that. It was a very outdated card that came with the computer.

Alright, I ran driver sweeper, and reinstalled the GTS 250 device, but I'm now getting this message:
"Windows cannot start this hardware device because its configuration information (in the registry) is incomplete or damaged. (Code 19)"

EDIT: Nevermind, I restarted my computer and the drivers reinstalled correctly. This still hasn't solved the blue screen, though, I started up a game to test it out and after 15 minutes or so it blue screened.

Thanks for all of the help so far

Alright, that makes sense. Thanks for the info.