No more AGP is good. PCI-E is a superior bus interface and it's been in production long enough now that you can find cards for it even in big box stores. The future is now.
Um... from the tech specs for the E510:
Slots
2 PCI slots
1 PCIe x 1 slot
1 PCIe x 16 (graphics) slot
So order an E510 with the no-cost integrated graphics, buy a PCI-E 6800 from Newegg (which will be cheaper than going through Dell anyway) and plug it in. You've got yourself an E510 with 6800.
Hate to be blunt, but you don't appear to know very much. Perhaps if you had told the techs what you wanted to accomplish, they could have helped you more.