First off, is your curent card AGP or PCIe? Probably the former, but it is important to know since there has been a relatively recent transition to an new standard for videocards. Here is the difference, and why it is important.
Apparantly, AGPx8 was the standard for the latest videocards, but it is no longer the case due to the PCI Express standard, which has double the data-pipes (x16). This means that cards that are natively PCIe would be able to achieve quality and speeds greater than that of AGPx8 cards. However, your motherboard either supports AGP, or PCIe. As such, that will narrow your options.
Mind you, I am not an authority nor expert on this subject. I only brought up the topic of AGP and PCIe to prevent any unfortunate mix-ups that could result in wasted money, due to incompatible motherboards or cards. Hopefully this helps.