Is this your motherboard?
Foxconn P4M800P7MB (Different brand, same chipset I think)http://www.dooyoo.co.uk/motherboard/foxconn-p4m800p7mb-rs2h/details/
If so, it appears you have an 8x AGP video card slot. So you'll be relegated to old-school technology for your video card. If I recall correctly, the fastest AGP video card made anymore (they've been phased out) is the ATI 3850? Make sure you get the AGP version, cause most of those cards were made PCI-E.
Also keep in mind, with your older Celeron processor, you may not realize the full potential of the ATI 3850 AGP card. It may be time for a system upgrade if you can afford to do so.