Monitors are measured diagonally from one corner to another. Basically it's a measurement of the diagonal diameter of the tube.
CRT tubes extend beyond the monitor case, as the edges get so distorted you can't use it anyway, and as a result a 19" CRT, for example, might only be 17.2" viewable.
LCD's, on the other hand, use every bit of the screen.. so a 19" CRT is 19" viewable. Most likely the 19" LCD is going to be bigger than your current monitor (although the widescreen makes it a slightly different story). If you have no real use for a widescreen, save your money and get a normal aspect ratio monitor.
Keep in mind, using a widescreen in Aces High will not give you an edge over your opponents. Yes the 'window' may be wider, but the height will be less, and as a result you'll need to be looking down to see some gauges and AMMO counters.. things you could normally see in the forward view of a normal aspect monitor.
My opinion? Dump the CRT. An LCD gives you a brighter, clearer, SHARPER image.. and also helps eliminate those awful headaches after playing for hours on end! The difference is truly amazing, even for my old, 'entry level' LCD.