i was going to get a 20" flat screen for my own dorm room next semester, but decided to spend a little more and got a 24" Toshiba 24AF41, $360 shipped from Ubid.
any one of those three brands, toshiba, sony, or panasonic will be excellent, it's all a matter of actually looking at the picture yourself and deciding which one you like the most. i love my toshiba so far, has plenty of inputs from s video to component to rca (the older sony 20" model doesn't have component i think), and the picture is great.
if you prefer online shopping, and you don't mind taking a leap of faith with a place like Ubid, you might be able to find a good auction for a flat toshiba 20" or 24"... it seems like they're the ones clearing them out or something. don't forget their shipping is like $80 or $90 on top of the auction price though.. ugh, but sometimes it still makes for a good deal.
also, goodguys.com might be good, they have this $25 off a $250 or more purchase offer going on, free shipping and no sales tax.
or you could get a 20" toshiba at best buy for $279.99... and have them promote their little extended warranty plan in your face.
i dunno too much about the new sony wega line... like the 20fs100 and 20fv300 etc., though hopefully their quality control has gone up. i've read a lot of people saying how they were having problems getting a wega that they liked the first time around, and had to make several trips to and from the store. but then again, it could just be people who are complaining that will bother to post online... it was enough to sway me to a toshiba though.