if ya think agp 8x doesnt have the bandwidth, then u have to wonder about this:http://www.tomshardware.com/2008/03/04/amd_780g_chipset/
what is the onboard video bandwidth? isnt agp better? now if ya look at the charts comparing agp to pci-e u will see no difference, but the pci-e is bi-directional and should theoreticly be over twice as fast, but it is not. there is little or no noticeable difference when u are using a 32bit operating system.
so your question, can be answered by your system settings or your video card drivers, ati agp card have a problem when fast writes is enabled. there is a fix.
the hd2600 is a good card and usually the problem lies with installation of the drivers.
try this adviser:http://www.cyberlink.com/multi/support/bdhd_support/diagnosis.jsp