I'm using my 3 years old pc for cad programs (VecorWorks, AutoCAD, 3ds Max etc.). As currently I do not have a budget to buy a totally new machine I have to stick with it but I would like to increase its speed as much as I can. Can you suggest the best graphic card? Also I would like to increase memory to at least 4GB. Or maybe 6GB? Not sure if it helps a lot as cad programs parasitize on a poor CPU.
No, DDR3 isn't faster. Don't you know the brand of the RAM (like Kingston, Corsair, etc?) you can keep the 2x1GB and add on the other slots. Do you use CUDA or in general GPU acceleration in any of your applications? If not, the video card is in there just for displaying the stuff and the CPU gets all the job done. You can overclock it to 3.0GHz and you need an aftermarket cooler in order to do that.