The 4850 is a pretty powerful modern gfx card, I don't see why you would encounter many difficulties with a 24" display (that'll have a native resolution of 1920x1200) with the vast majority of games as long as you don't overcrank the AA/AF settings too high.
22" displays will mostly have a resolution of 1680x1050 (except for the occasional 1920x1080 "full hd" resolution screens you get at this size but text size would be quite small) and therefore put less strain on the graphics card allowing you to turn up the AA/AF image quality without too much effect (where you could really do with a couple of 4850's in crossfire if you were to do this and maintain a high framerate on a 24"+ screen).
Remember it is easier to buy a second 4850 at a later date on the cheap when the enevitably (sp?) will come down in price when new tech is released and run this in crossfire, rather than getting a 22" screen today and wishing you'd have bought a 24" screen instead 6 months down the line.
Also the 24" screen resolution of 1920x1200 will allow you to use "Full HD" movies on it (1080p) from a blueray disk that, again, will almostly certainly drop in price over time.
I normally change my screens in cycles of 4 years and I've just today received a Hazro HZ26Wi (yet to test it) but I wouldn't want to consider changing again until 2012 (hopefully SED screens will be the norm by then!)
If it was me, I'd go with a 24" screen purely based on the fact you are more likely to change graphics cards more frequently than monitors
