Try looking at the problem another way.
How many hours a week are you "waiting" (not being billable) for the computer?
On the average how many hours do you lose because of slow rendering times and/or crashes or other down time?
Multiply this times your billable rate and that is how much time in $ you are being unproctive because of inadequate hardware.
If a new computer costs $5000 and you're billing rate is $65 an hour, than a new system will pay for itself in 77 hours. After the 77 hours you will be making $X more money a week.
I'll let others comment on the hardware specifics but even to me you are asking for just a less out of date computer.
Of course, I'm not paying for the new system.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"