Well, there are a lot of things that I'd say will make your life cheaper. First off, why do you want a widescreen? First of all, if you look at any work that you may be doing, it is always shaped like a piece of paper, which is in the opposite direction. To me, it seems stupid to have a really wide monitor and then display an upright document on it. The web is also formatted like this, as are most programs/chatting programs, etc.
Second of all, widescreen monitors generally have oddball resolutions, so when it comes to gaming, you will always have difficulty getting the game to conform to the native resolution of the monitor, which is very important because the last thing you want is for the LCD to be interpolating your images. That never comes out right.
Third, why are you aiming for a screen with a higher resolution? I'd say that the lower the resolution the better? Why? Many reasons. For one, the lower resolutions offer a nice text size in my opinion, and anything of greater resolution will result in too small an image unless the screen size goes up accordingly. Apart from that, you are planning on keeping your system for a long time, and the native resolution problem kicks back in. The higher it is, the less likely your system will be able to run games at that resolution in the future. Stay low, and your chances are greatly increased.
As far as size goes, it's entirely up to you. If you feel that you'd benefit from a larger screen, then go for it, that is not for us to decide. Lots of people are happy with sticking with 19" screens and find it somewhat wasteful to increase the size, others find it more immersive. Just look at monitors in the store and make up your mind on what size you want/need; it's entirely up to you.
Hopefully this can clear up a few things and help you with your final decision as to what monitor to purchase, and may help you from burning a hole in your wallet. One thing to always keep in mind, though, is that no matter what screen you get, you'll probably be happy in the end anyways. It's always a good way of looking at things. That way you always get the best bang for the buck. And who knows? Maybe if you save some money on the monitor, your next graphics card upgrade will come sooner than you think ; - )