There are 3 main elements to the choice.
1. the size of the monitors
2. the resolution of the monitors
3 cost versus performance
there are then some secondary issues such as class of TFT, bandwidth capability, refresh rate, dpi, contrast levels, maximum brightness, emission standards, response times, connection standard (analogue or DVI) and dot pitch.
Complimenting these are the secondary functions which may be offered ie. USB hub, integral speakers etc.
A 21 inch TFT screen which has a 1600x1200 native resolution will be four times (at least) more expensive than a single 15 inch which supports 800x600. However, the 21 inch (in this example)would be able to display the same data as four of the 15 inch screens and would generally be of a far higher specification in other areas.
The big but with TFTs at the moment is very few manufacturers or retailers are prepared to certify that the screen you purchase will be error free. The higher resolution screens are more prone to manufacturing faults.
Check out
http://www4.tomshardware.com/display/index.html for some very good articles on TFT/LCD screens.
So, before offering any assistance it may help if we had a better idea of how they will be used.
What are the resolutions of the current screens?
Are there any special requirements (USB hub etc.)?
Does your graphics card support the TFT of choice (resolution and connection)?
Unlike Skim I find it hard to condense 30 years of experience in this area into a couple of lines