USB capture works....

...but the frame rate varies greatly depending on the resolution and content! - so can sometimes be unreliable
see quoted frame rates for different models:
DVI:
http://www.epiphan.com/products/dvi-frame-grabbers/
VGA:
http://www.epiphan.com/products/frame-grabbers/

The 'DVI2USB' model offers the best value as it can capture either DVI or VGA at a reasonable frame rate (although not always perfect).
http://www.epiphan.com/products/dvi-...bbers/dvi2usb/
available in the UK

However, if you are looking for a laptop solution, and you have a card slot - some users have had success using a PCI-Express card box with an Active Silicon LFG4 card
http://www.magma.com/pciexpress.asp

Not tried this or compared the capture rates or latency - but I have heard that it works...

And finally....
DO TEST!!!

Simon