System Requirements for Fusion. Download a trial and test it out with 1GB on the VM and see how it runs. If you see that the VM needs more RAM or that OSX is battling, then it'll be a good idea to add more RAM. If your CAD usage is going to be demanding and/or if it's a production machine, then I'd go for the upgrade.
If you do go with Fusion, I'd recommend the non-Pro, i.e. cheaper, version for your purposes.
One further thing to consider. If there is an appreciable difference between the Mac and Windows versions of TurboCAD, then go with Windows. If not, stay native. If needs be, contact the makers of TurboCAD and get the low down. I suppose one other item is whether you need any other plug-ins, such as CAD/CAM - would they be OSX compatible?