Agreed ^ He is simply not making you a very good offer here.
Too much for quite frankly, a dated system on a dead end platform. Socket 939 and Standard DDR memory, an old Operton that likely has been overclocked for most of it's life. It's a really slow processor by todays standards, with the 600mhz overclock it is better, but still won't hold a light to a modern processor. A couple of 7800's in SLI? A single $100 card will spank them. It's an old gamers rig that's time has came and long gone.
I would not pay more than $150 for the whole thing including the monitor, or you are better off to start shopping for something new, with a warranty.
For CAD, you don't need much graphic power at all. You will want a quad to make it really run smooth, and on-board graphics is good enough for what you are doing.