Many have had issues with the 8500dv with firewire...is this the card you have??? If so then go 
www.rage3d.com as they are more ati tech based answers there then the 3dmark crowd around here in the video forum...
I myself do not use the firewire port on the card yet, but my port on the audigy has in the past worked like a charm....
What drivers are you using?? WDM?? Catalyst?? Control Panel???
May try a firmware update...go to atitech.com and see what is the latest and then at bootup pay attention to the bios number for you vid card in the upper left hand corner before cpu and mem post...