1. Faster load time for drawings on open and save. I don't know what specific subsystem is best for this, but that is one task that can be really slow.
2. Faster load time for regenerating drawing (am guessing this would be CPU for recalculation and memory for the same reason).
3. I don't know if AutoCAD can off-load 3D calculations for display purposes to a graphics card, so I'm not sure if a high-end 3D card is worth the money. If most of the rendering calculations are done via software or the CPU, then a quality 2D card should save some moola.
4. Whatever he does, shell out the bucks for the latest version fo AutoCAD that JUST got or is JUST about to be released. Drawing load and save times as well as file size of the .dwg files have all been cut by 35-75%. Many are calling this the biggest upgrade since R14.
I've got some technical CAD specs around here somewhere, I'll see if I can get u some more specifics.
I'd opt for CPU and memory, though CPU's are more easily upgraded (within a family of course), so more memory and FSB = better. So I figure a 400Mhz FSB and some screaming memory, and gobs of it, at the cost of some CPU will be better, that is IF he isn't averse to upgrading his CPU periodically. My $.02 worth.
P-X