Hi.
You can look busid with lspci, if your videocard is in PCI slot.
I had to add BUSID too to XF86Config-4 to get X work.
Eg.
Output from lspci:
0000:00:0c.0 VGA compatible controller: 3Dfx Interactive, Inc. Voodoo 3
(rev 01)
Now add BusID to XF86Config-4:
BusID "PCI:0:0c:0"
Of course, use ID of your system.