very good choices. probably budget of 1k to $1500? appears to be the realm depending on hdd and other stuff. 4870 might be too expensive for that budget but it would be a great upgrade to do if you can. probably don't want to crossfire since it would be x8/x8 if you do so going as good as you can on a single card would be beneficial.
also have you gone and physically looked at monitors? the reason i ask is that some people once they actually look like 22" 1680x1050 just as well. and at that res you can do more with the 4850 or save some money to allow you to get the 4870 and be pretty set for a long time. but other people i know can really tell a difference and swear by the 1920x1200. so if you haven't may want to compare.