|"Ram: 960 MB"|
You probably have 1GB RAM with 64MB used by the onboard video. How are you coming up with 256MB for the 6150?
"Processor: AMD Athlon 64 x2 Core 4200+ 991
I have no idea what the 991MHz is? The 4200+ runs at 2.2GHz. And you should be getting DDR2-800 to go with it...that's assuming you don't have an older system that uses DDR400.
As for the video card, I'm with kuwese. The Radeon HD4670 w/GDDR3 is the best card you can get without having to be concerned about your power supply.