you wont find many 'retail' computers anymore with agp slots (or any grfx ports unless you want to spend some G's on gaming systems).
just find a new motherboard and build your own.
www.newegg.comif you want to upgrade i suggest the ASROCK 939SATA2 (i forget the exact name) it has agp and PCI-E (ie its pci express not just pci). aswell as socket 939pin amd aswell as an addon board for AMD's new AM2 processor (and ddr2 memory).
not as well performing as the other competition, but for upgradablity alone its great.