My graphics card is an nVidia GTS 450 which will only run two monitors at a time, but I'd like to add a third one. Will I be able to do that using a USB to DVI adapter or would the limit still be there? Also I've been told the video quality is really bad with these adapters, so is the quality of the video output by these adapters okay for things like web browsing and working on office thing? And is it worth the $60 or would I be better off just buying a cheep second card?
Everybody's needs and setup are different, but I'm using a USB-to-DVI device right now for a 3 monitor setup and it works great. Mine is a few years old, but it's basically the predecessor of this adapter. I don't think I'd game on it, but for work purposes the graphics are just like the other two monitors on my system, even for video.
Unless you have a compelling reason not to add another card (for example a lack of suitable slots), the cheap second card would be a better idea.
The issue with USB video devices is the lack of bandwidth - they've often have been described as not being able to handle full motion video, but your mileage may vary.
If you intend to do mostly text, it may be an option. If you're doing say video, or stuff like that, the proper graphics card is a better idea. At equal cost or even slightly higher cost, the cheap video card would be my pick.
Cheap second card if your system supports it. I used the cables, and they are terrible for anything that requires constant refresh. For emails and web browsing, it's perfect
Just to give some facts/figures here, a uncompressed full-HD stream is about 3 Gbit/s (at 60 fps). USB 2.0 has a theroetical peak of 240 Mbit/s in one direction.
So, a USB 2.0 adapter can't achieve full frame-rate. Even if compression and partial updates are possible, it won't be pleasant for watching hi-def movies.
Also note that these adapters seldom support 3D or probably even reasonable hardware accelerated 2D drawing, so most gaming is usually right out as well.
In my experience, drivers for these USB adapters can have very dubious quality. It's getting better, but if you can afford a cheap (used?) but proper GPU, go for it. (If it's the same brand/make as your primary card, it's probably a win to avoid compability issues)