Programs like Autocad will run much better with 8 GBs (or higher) RAM. The 'regen' command will work faster, and circles will be represented as circles and not polygons. The same goes for CPU frequency; the higher the frequency the better the program will run.
I use a first generation (i7), and also a third gen i7 to run Autocad. The speed is not an issue. The computer is faster than my design thinking speed.
Before all this I started out using Autocad on a Pentium 4, 1.6 GHz (slow) processor. Even here, the computer was faster than my brain!
For such work a large screen is essential.
Regarding your question, take the above into account, try to get 8 GBs of RAM and look for the best priced computer. You will need 320 GBs or higher hard disk, and an external drive for backups.
I am currently running a core 2 duo, will it be a sufficient upgrade?
Also I think that rendering using sketch up etc takes a lot more time than normal autocad drawing. rendering time for graphics can be in minutes.