Yeah sounds to me like your graphics card is going bad. But I guess it could be a power supply issue too. (I'm sorta new to this stuff LOL)
I had a similar issue with my ATI Radeon 200 Xpress (way outa date) graphics card. The only difference is my screen would go white and sometimes have colored up and down lines on it, and I would lose my sound too.
My computer drivers were out of date and it was refusing to let me update them (said system requirements were not met "pssh bull"). I bought the video card your having problems with right now and I'm not having any problems yet with it... except my computer sometimes unexpectedly restarts itself (and that might not be related to graphics card :-/)
If you open up your computer case (not on a surface which may cause electicity, use table), look at where your power cord is plugged it and you should see a big box thing... that is your power supply... It should have a label on it that displays it's watts... make sure that they are 300W or greater like the graphics card requires. I would also suggest looking to see if your graphics card has any driver updates (installing these might solve the issue)... The cd that came with the video card if you have it will look for updates.
Not sure your power supply watts are going to be the issue though, because my computer is 5 or 6 years old and before I upgraded my power supply (mysellf, it's not actually that hard you can find instructions online for your specific computer), my original power supply was 300W. Therefore, I doubt that you have under 300W unless your computers very old.
Hope this helps
... and GL