It really depends on how well it's working for what you do with it. If the performance is acceptable it's always better to wait for newer and better and/or cheaper to become available. I'd replace your monitor first, then see how your performance is. Replace the 4850 if it isn't satisfactory. A place you can check benches http://www.anandtech.com/bench/Product/177?vs=164
The same performance as the 4850 would be the 5750 or the GTS450 (@$115). It would make more sense to spend an additional $10 or $15 to get the GTX460 768mb or the 5770 (@$130) for a bit better performance. To get noticeably better performance then the 4850 would take a bit more money. Here's a hierarchy chart, check the whole article. http://www.tomshardware.com/reviews/best-gaming-graphic...
yeah well it depends on the make and model of screen but generally speaking for 20" you will either get 16X9 or 16X10 which is basically the same, a 21.5 or 22" or greater is generally 1920X1080, really I wouldn't worry about it because 1650X1080 vs 1600X900 is very little difference visually, just stick with the 16X9 until you are ready for an upgrade to 1920X1080 or greater, but keep in mind with a bigger res, you will require more gpu horsepower
i went from a 4850 to a 460 768mb... the difference was pretty good, especially if you overclock. I have my 460 from 675 to 920mhz! , I am very close to gtx 470 benchmarks on metro 2033 at 1680by1050 (on my 20" monitor), and on 3dmark 11 I am also close to the 470 and 6870.
If you have the money though the card i recommend you to buy is the 6850, which I hear can be over clocked to 6870 levels. If you are short on cash, the 460 768mb is very good, and noticeably better than the 5770, 4850, 5750, gts 450 despite what the toms hardware charts say.