USB, 1 or 2, is not fast enough for video. Firewire is the best plug-in digital video interface, it's designed to keep up with video.
If you must convert analog to digital, use a PCI bus video card that has hardware compression. Software compression can't keep up with video. Also hard drives will have trouble keeping up. RAID arrays may be fast enough for real time captures, but they have their own drawbacks.
The Math (tm)- 720x576 at 25 fps worst case (8 bit RGB) may need as much as 31 megabytes per second to avoid dropping frames. Hard disk drives can easily have a write rate below this. DV compression reduces the video data rate to about 6 megabytes per second, a rate most hard drives can keep up with.
A Panasonic Mini-DV camcorder with analog pass-through combined with Firewire is sometimes what a friend of mine uses for this in his video business. It also does jitter filtering at the same time.
MPEG2 compression at 9 Mb/S will only need about 1.5 Megabytes per second. This is why I use a Hauppauge PVR-250 PCI bus card for all analog conversion.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70