You have a respectable AM2 CPU and 2GB of RAM. Can you get a mfgr & model number off the power supply? Or get the data off the side panel of the PSU? It should look something like this:
We are most interested in the +12V and any +12V MAX info.
Mfgr is FSP group Model is ATX-400PA
There is no problem using an ATI GPU in a Nvidia motherboard or Nvidia GPU in an ATI motherboard.
Do you have a budget in mind for the video card upgrade?
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Also can you list your favorite games or the games you're thinking of getting? Some video cards handle certain games better than the other brand so that can make a difference.