A definitive budget would be usefull?Although you have typed a mass of information make/model/type and size is really all that's required.
Are these your components? Mobo CPU GPU am I correct in saying you have 2GB of PC 6400 ram? I am not surprised it is showing its age particularly using Autocad. So if you could post a budget, I'll have a look for suitable components, I think a complete rebuild would be the way to go keeping your current rig as backup/spare or break it up and try and flog the parts on EBay.