The best cards for AGP were the AMD Radeon HD4670 and the Radeon HD3850. NVidia discontinued their AGP line a generation or two earlier. I think the best NVidia card might have been the 9600GT or the previous version 7800GS
You also need to identify which model of Dell GX260 you have (mini-Tower, Desktop, etc...) as a full height card will not physically fit into the desktop version of the system.
-Wolf sends