Basically PCI-e has replaced AGP for the graphics card socket. PCI-e and AGP are not compatable.
This has only happened within the last year or so and so your motherboard has no PCI-e slots, you do however have normal PCI slots (not sutable for graphics cards, just network cards etc) and an AGP slot.
Therefore you have to but an AGP graphics card unless you upgrade other parts of your system (which i doubt you want to do)
The x850xt AGP is a good card to get as it has a good cost to performance ratio iirc.