that sounds like a Dell build or something of that nature.
Yes you can use an Nvidia 8 or 9 graphics card with that system, but I strongly suggest upgrading the power supply, im sure whatever it has is garbage.
These two components would be a huge upgrade for your system: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127362
530 W power supply.
Also, if you are running Vista, get two more gigs of ram for cheap!