That Gigabyte mobo is an old intel 440 BX mobo for celeron/p2/p3 cpu's.
Before you said that the screen became totally black when using your card on the Sis 645 mobo. Those modeswitches occur when verifying all modes for your monitor. Something strange is going on. Since the xfree86 log file was far from complete, I suspect that the system crashed or something like it, else it would have finished writing to your xfree86 log file.
Some times it happened to me that I had black screens. (in the past using old drivers) If you can reboot using ctrl-alt-del the system isn't dead. For me doing ctrl-alt-f2, ctrl-alt-f1 (just console switching) gave some screen again. Perhaps you can see the real errors then.