have you installed the VIA 4-in-1 ? That fixed my issue on my old Tyan board. The VIA southbridge doesn't play nice with ATI for some reason.
Ah ha. its a power issue. if you haven't found it yet, The ATI9800 and 9700 chipset disable 4x agp when they see the a VIA southbridge. This causes the default to be 8x which draws additional power through agp slot. the 4x can't handle it so you see nothing, no post. Try setting the Bios to 2x AGP and see if you can reseat the ATI and run it. from there you should be able to patch the ATI driver and VIA 4-in-1
I asked for computer help on mongrel but I got carl'd instead