My experience backs up Millsy's thoughts - i.e USB connections are used for capturing still images only. Some cams offer video capture via USB, but this generally produces poor results - slthough USB 2 is technically quicker than Firewire, the standard adopted by DV is the latter and gives the best quality.
If you've got money to spend and you're looking for high end results, you should compare the offerings from Canopus, Pinncacle and Matrox. These offer video editing cards on or below the £1,000 mark. My site has so far concentrated on budget editing, as I haven't had the opportunity to look at the higher end of the semi-pro market.
However, there's a few UK site that specialise in video editing equipment that provide useful info for customers (and window-shoppers!)
these include
www.sub.co.uk and
www.dvc.uk.com
They don't necesarily offer the best prices, but the latter gives a good overview of all the cards on offer