most newer LCDs have much improved response time. Another thing to consider would be viewing angle. The voices in my head told me to say hello to the voices in yours.
Since my cabinet is built, and I have a 23"h x 24"w monitor area, the only way to maximize that is to seek out the older 21.3" 4:3 LCDs that are 1600 x 1200. None of them are as fast as today's panels, but putting a widescreen LCD in my cab, either horizontally or vertically, just wouldn't look good.
Some people have their head in the sand on this issue, but having a display with minimal input lag is very important for gaming. The game will feel a lot more responsive.
Also, keep in mind that response time is not a measure of input lag. An LCD with a very low response time can still have a lot of input lag due to image processing.
Input lag is rarely listed by the manufacturer. Some say they don't want you to know, but I think it's often that they're not primarily concerned with gaming applications. You need to find a measurement from a reputable source to determine if a monitor is suitable for gaming.
http://www.displaylag.com/display-database/
http://www.displaylag.com/what-is-input-lag-the-breakdown/
Great points and links, thanks. I'm going to try a Viewsonic VP2130B, based on the LTM213u6 panel. It's the same panel as the Samsung 214t, which others have reported as being good in MAME. If it's no good, I'll go with a "22 inch" (actually 20") crt computer monitor.