Your goals are unrealistic considering your budget. Buying used cards is a complete crap shoot. In my experience it's usually not worthwhile since there's a good chance the card will be DOA. I've actually purchased a card that was "tested and working" and when it arrived it clearly had toasted/leaking capacitors (which I would have seen had I looked closely at the listing photos). I've had similar experiences with two other cards.
If you're certain that the 4850 is in good working order it's not a bad deal at all, but it does require a single 6 pin PCIe power connection. If I were you though I'd up my budget a big and just get a brand new AMD Radeon HD 6670 which is the best card you can get without using a 6-pin power connector and it offers performance nearly equivalent to the 4850 you list.
EDIT: Asus HD 6670 with GDDR5 memory that's in your budget.... http://www.newegg.com/Product/Product.aspx?Item=N82E16814121442