Hello peekaboo. The difference in the cards is because of the GPU/VPU memory DDR DDR2 DDR3 and then you have the choice PCI-E or AGP.
Here is a list fof the Radeon cards and details about each
http://www.ati.com/products/gamer.html
Can you tell us does your motherboard support AGP or PCI-E as this is important Loz.