No, the HD4850 won't accelerate your 3d renders. If you're going to use AutoCAD then the best you can get is an Nvidia Quadro graphics card. Nvidia's Quadro line is designed for accelerating 3d renders especially in AutoCAD, 3DSMAX, Adobe Creative Suite 6, Dassault Systems Catia, Dassault Systems Solidworks, Siemens NX, Avid Media Composer, PTC Creo, Parametric 2.0 and Pinnacle Studio 16. If you're a beginner or you have a limited budget get a Quadro 410. If you've got a bit more money to spend then get a Quadro 2000. If money isn't a problem then get a Quadro 6000. In the UK the Quadro 410 is 120£, the Quadro 2000 is about 400£ and the Quadro 6000 is anything from 3000£-8000£. If you want the AMD equivalent, the entry level CAD certified card is the V3800, the midrange card is the V5800 and the high end card is the V8800. I can't tell you the prices of the AMD cards because they are much harder to get.
I also do 3d modelling