That depends on the lcd tv ! Some lcd TVs (f.ex. 20") have low resolutions like 640x480, but most larger TVs have 1360x768 computer resolution. Stay away from those with low resolution. 1360x768 pixels on a 26" gives you a dot pitch of 0,42 mm. Perfect for games and movies with a crystal clear computer picture ! The brightness and viewing angle on a TV are also usually larger than on a computer monitor. (A computer monitor is better if you do computer work or need a faster responce time for gaming with those RTA monitors.)
If you want something larger than a 19" LCD computer monitor then my recommendation is a 26" lcd TV. But be aware that there is a enormous difference in quality and input options on different brands. Choose a TV with max 8ms responce time, more than 1000:1 contrast, Component, VGA, HDMI and Scart (if you live in Europe or Australia) and a descent scaler chip.
Note that the HDMI/DVI input on many TVs are not always compatible as a computer connection. Usually the best connection is through the VGA (HD15) even though this is a analog input.
I have a Samsung (something) 26" with 8ms, 3000:1 and I am very satisfied with it... I used to be addicted to CRTs (10 TVs and 2 pcs. 27" arcade monitors). If I would choose a TV for Mame only I would say a 21" TV with RGB-scart connection, but things have changed...the LCDs are here to stay ! (At least for a while !)
