Bigger is always better, don't let the girls tell you otherwise . I just upgraded from a 19in round monitor to a 22in fp2141 flat AG. Takes awhile to get used to it and since the screen is so big the corners aren't as sharp as center of the monitor, but it definately beats anything else I have seen. Also you need to take in account I am the most anal person in the world when it comes to video quality. I once spent 4hrs straight watching test patterns and accessing service menus to get my bigscreen tv looking perfect. And then of course I had to redo the convergence every 3months which takes about an hr since it just gets a tad bit off over time.
So when I say it isn't as sharp one the edges, I say that with a big stick in my butt .
If not, I would probably say go with the dual screens. I played around with my friend's dual 19" screens and it was kinda cool, but annoying b/c anything that normally comes up in the middle of your screen was split between the two.
single huge screen is better, but seems like your happy, very happy , i running on ONE 17 inch and i'm happy. But i'd take the 21 if i were you since it really gives a gaming advantage.
How about a 21" and a 17" dual??? That's what i'm running, i have a Viewsonic 21" CRT and an IBM 17" LCD. The size is awesome, but i'd rather run a dual display that is slightly smaller then a single display that's bigger. Thats just my opinion, but either way you can't go wrong, they've both got major benefits. If you get the 21" you can always add a 17" down the road for a dual display when you feel like it.
if you have a decent program or video card driver you can tell the computer where to send stuff when it opens. The only thing I hate about dual monitor set ups is trying to get programs like windvd to stay running when your playing a game or to get it to work with you 2nd monitor (ultramon can fix this a lot of the times by swapping which is the number one monitor but why can't they both show video?) so thats my major annoyance. I agree with most people: Gaming go big/ surfer dual may be better but a 21 is WAY better than a 17.