I'd get a nice 19"
The chances of you fully utilizing those two monitors is slim as you only do overlay on one of the monitors. Also I believe WinXP has better dual monitor support, so if you don't have that then once again I would suggest a single monitor.
I have a 21" monitor, I used to have 17". There is a big difference
Web pages look nice @ 1600x1200, I can have two 720x480 video windows open with 10 video tracks expanded :p
So once again, I would just get a nice 19" or even a 21" if possible. Also a flat screen monitor helps prevent Moire Patterns(ugly circular rainbow lines that looks like crap).