USB to Video adaptor

[#1]

I want to add another monitor to a system and only (currently) have USB ports available.
Has anyone used USB>Video adaptors? any recommendations?

[#2]

Can't you replace you video card with a "dual head" one? Definitely a better option.

[#3]

All the ones I've been able to discover are Windows only.
My research also shows they are relatively low res and require software to be loaded, so its a long way from a plug and play solution.
My recommendation would be, don't do it unless you really must, and there is no other solution, oh and of course, you must be running Windows and don't expect any FPS games to run on it.

[#4]

@HB and 44Devon
Thanks info, may have to think again, might need an extra PC
I've used 'not very hi-spec' PCs that I use to monitor events, Each PC currently has 2 x EISA dual head cards feeding 4 x monitors, I want to add a 5th monitor to each PC.
(see picture attached, 3 PC's running 12 monitors, Top row is currently being swapped out).
I've investigated a 3rd dual head card, too expensive, means swapping out the mother board.

[#5]

Hey there Journeys,
Have you considered DVI splitters?
Such as this: http://www.cableuniverse.co.uk/lindy-dvi-dual-link-splitter-cable-2-way.html
Or i'm sure there are some VGA ones floating around the internet
Regards,
Matt

[#6]

Erm, that will result in 2 monitors showing the same image.  But you do remind me of the Matrox dual heads which I've used, again you need to load software, but they work quite well.

[#7]

@Matt thanks, have DVI splitters for another application 44Devon is right duplicates the image on the second screen.
Was looking at USB>Video as a low cost solution something like this http://www.amazon.com/External-Multi-Monitor-Video-Adapter/dp/B002FJQXXC
@44Devon, I'll have a look at Matrox Dual heads, thanks