On Boxing Day last year, I bought a new monitor. A 19" Samsung SyncMaster 950p. I love it!
For a long long time I've had two video cards in my machine, Matrox Millenium G200s (PCI, 8MB each). The 2nd monitor I had connected was a 17" Daytek. (Before I had the 19", I had an ADI 15" as the secondary monitor) It's been really flaky for well over a year, with various guns cutting out sporadically, requiring me to pound on the top of the monitor case to get the colour back to normal. :)
Over the last week or so, that 17" monitor got worse and worse to the point where it was really hurting my eyes. Since I do a lot of development, bad monitors are bad business.
So, yesterday, I picked up another SyncMaster 950p. I'm in pixel heaven! Both are running at 1280x1024 @ 85Hz with 32-bit colour. I could run them each at 1600x1200, but I'm happy with this resolution.
I'm really enjoying working with these monitors. I feel more productive already, after only a day.
Before buying a 21" monitor, consider 2 17" or 2 19" monitors instead (assuming your computer can run multiple monitors - most modern computers can now). Way more total screen space than a single 21" monitor, and if one of them should happen to die, you still have a monitor left.