yes, you probably would notice the difference.
currently i have my 19" LCD and my 17" crt together in duel setup.
if i put a window so its halfway on both monitors, the difference in them is obvious. the crt looks dim, faded and blurry compared to the lcd.
as for the resolutions, it'd be easier if windows etc could be about the same size on both monitors. with my 19" lcd at 1280x1024, i have my 17" crt set at 1152x864 which makes windows about the same size on each monitor.
overall, its best to have 2 of the same monitor in a duel setup. that way they will have the same brightness, contrast, etc so your eyes wont have to constantly adjust when you look back and forth between the monitors.