I would strongly reccomend in favor of two 19" monitors over a 21".
I'm just about to send my 21" CRT back to sony for a warranty repair,(less than 2 years old) and I had a previous 20" die in less than 3 years as well. If I could go back and do it over, I wish I'd bought a good 19" instead. I may sell the 21 after I get it back, before it dies again, and get a good 19".
Weight & heat production are also very real issues with the biggest of the big CRT's. I had to get a stronger desk, because the older one was bending.
In short, after using a couple big CRT's and now a couple big LCD's, I don't plan to buy a CRT over 19" again
Bang-for-the buck, I'd take a NEC/Mitsubishi Diamondtron flat tube over anything else out there right now, particularly their 19" "Superbright" monitors that just came out this summer.
Remember that it's not such a bad idea to get one, and then save up/beg/ use holiday/birthday to buy a second one later...
Good luck,whatever you choose.