Out of those three cards I say the 4850 is you best choice. If you had a little extra scratch you could get the current gen 5750 or 5770 that would perform slightly better then a 4850, but as it stands with those three you'll win with the 4850. I used it for CAD and 3DSM / Maya / Lightwave just fine.
OK, thanks people...price range came from having to factor in a new psu and case! thank nrnx, really useful to know it will run well with 3DSM especially!! Any suggestions on a good price/perfromance psu to go with the 4850?