When it comes to CF, i suggest u to CF 2 identically cards, so u already have 4850 then add more one 4850, u will get a high jump performance with that setup...
Make sure your PSU can handle that cards...
You can CF a HD 4850 with HD 4870X2/4870/4850 and 4830 so you can't CF it with a HD 4650.
Between them the best thing is to CF a HD 4850 with another HD 4850 since they are the same card