A capture card should let you record from a VCR to the computer with out going via DV, the main advantage is you can use different encoding other than DV. ie if you need to work with uncompressed video. You can also put out to a VCR from the card or if you are using FCP you can see the video output on you Video monitor (TV)
On the G4 use can use a
DC 30 card with
this driver it works very well, the the DC 30 will not fit in a G5.
For the G5 you might want a
x-fuse
You will also need Final Cut Pro to use a Card, iMovie & iDVD only work with DV.
As Sdashiki said if you need to record of TV / VHS you should be able to record through the DV camera.
Sorry G5Unit but i think you are thinking of some sort of screen capture device.