I tried Time Machine on a non core graphics card once. Don't do it, its so slow.
All the Mac would do is sit there and backup. It's so slow now running OS X 10.3.9 that it's already painfully unusable. With that in mind, my main concern is that the backups would take place and that I could restore an entire volume if the hard disk would fail. I'm not really interested in using the interface that lets you retrieve a single document using coverflow technology.