If at all possible, go for two 19"+ TFT screens. After you get used to the screen real-estate you will never miss your old monitor.
I've been using
two L1910S LG monitors and would never go back to a single monitor setup. Especially since I frequently use several programs at a time, and the ability to see them all at once without switching between windows is great.
Even a 2x17" TFT setup would be very good, and wouldn't cost much more than a single 21" CRT.