To add:
Although it is not strictly essential, I recommend having all monitors of the same model as well as both graphics card the same and each card having 2 identical outlets (e.g. both DVI). Otherwise the image will look different on each screen. For a while I used 2 monitors of different sizes, ratios and resolutions. These particular factors didn't really bother me much though. However, what I found most annoying was the difference in color balance, brightness, contrast, etc. that my eyes kept having to adjust to when shifting focus from one screen to the other. No amount of tweaking would get them anywhere near to looking the same.