Eric,
I to am an Exhibit Designer using VectorWorks and RenderWorks on a G4 500 Mhz, with 512 Mb Ram and 700 Mb virtual Memory (Hard Disk acting as ram), I know its a different system but if I had a (which I am getting a lot latley) 15mb file, I reckon to do a hidden or dashed line render , it too would wipe an hour out of my day, base on previous attempts.
I tend to model every single thing in a stand up to real life detail, so when it comes to working drawings and details, I can lash out isometrics and sections, elevations etc in a jiffy.
however to do a full renderworks rendering on average a 15 - 20 mb file at max resolution should pop out in 30 mins, which is toleralble, but the hidden line work naturally takes more because I assume its vector based.
Have no fear, maybe invesigate adding another processor? not sure if it'll help but ask the guys on the help desk.