504? do u mean 512?
look at your motherboard, there should be a couple of white slots (pci). tell me if there is one above all of the white ones. if theres is, it is slightly smaller and is more to the right than the pci slots. if you have this, its an AGP slot and allows ana gp graphics card which should be great for gaming.
Gigabyte GA-7S748-L: AMD Sempron 2800+ 2.0ghz
Viking 512mb PC3200 DDR RAM
Maxtor Plus 80gb 7200rpm
Ati Radeon 9600xt 256mb