4850 is still a decent enough card, i sold mine at the beginning of the month since there is nothing i want to play until Mafia 2 comes out and i will really need an upgrade for that game anyway. It's a shame you don't have access to the Member's Market since there are quite a few around. Mine sold for £45.
I think the real question is, what are your expectations for the card, what games do you play and what resolution etc...? I got very good performance in Batman, Mass Effect 2 and Bioshock 2 @ 1920x1080, but there are many other games where the 4850 will show it's age more.
I know graphics prices are through the roof at the moment, but it might still be worth saving for a bit and going with a more expensive card if you plan on keeping it a while.