Well from that site there are 2 types
one just lets you hook it up to an external display and are common, they do not improve performance at all if anything they may even lag or just slow the machine down using it.
the other type, that personally I have never seen, but one of those can improve performance but costs are not cheap according to the site. (the cost is expected)
reading more about USB solutions though informs me that they do not produce much of a boost, not unless you have a laptop expansion slot (most don't now) or a docking port (PCI Express 2.0) that proves more speed compared with USB 2.0 that is rather slow compared to what is needed for clean graphics processing
It would still be more cost effective to get a new desktop rig that can at least be upgraded from time to time, something that I want to do myself at some point
Docking stations such as
http://www.asus.com/Graphics_Cards/XG_Station/ should give a good boost again not price effective and only works with:-
Compatible model list:
Asus models
W5F, W6F, U5F, U1E, F3SA, F3SV, U6S, R1E, U3F, Lamborghini
Not many Laptops/Netbooks have PCI Express, most just have USB that can be used for external devices