the 8800gts will power two monitors, i would probably got for the 22" (lower maximum resolution= better framerate 1680x1050 opposed to 1920x1200)

as for which one to use as your main the choice is yours, I would personally go with the 22 for the main and the 19" for the slaves ( you could always use PiP to swap screens if you needed to perform any tasks on your slaves). as you would spend most of your time running from the main screen

you could always swap them over once you have the additional screen, if you wanted to try the slaves on the 22"