I'm currently wielding 2 4850 witch i'm still very pleased with. They play all (almost) with full eye-candy enabled at 1680x1050. I'm not changing monitor currently. But i have an itch. And i'm itching for a 7850 or even a 7850 CF. My board doesn't support SLI, so CF is the way to go. I'm think of buying one now and one later.
Would i notice any difference ? Or it is just burning money ? I have a X4 955 Currently that i'm not upgrading soon.
You would probably not see much performance, if any, from going to a 7850. You would see better noise/temps, and make sure you get a non-reference board. A 7850 CF would blow your current setup out of the water