The 9800GTS would be best if there is actually an AGP version of it. I am nearly positive that no companies produced an AGP version of it. But if you found one then good for you.
Your motherboard is only compatible with AGP 2X/4X video cards. To the best card you can buy new is the HD 4670. The HD 3850 would be faster though, but I'm not sure if it supports AGP 4X.