VHS capture is real time capture of standard def video. Unless you are using some processor intensive compression in between, capturing the stream should be fine. I was capturing VHS videos quite a lot with my old Pentium 3 computer in the days, so I doubt a core Duo Mac will have any difficulty. The question is, is the hard-drive in tip top shape. I would think that that would be the bottleneck most of the time (ie. failing drive, disk errors, etc).
For HD, I'm not sure (although straight capture shouldn't be a problem as long as the bandwidth is enough and the drive is fast enough). But don't expect any fast encoding performance.