Most of the system is on the low side of being able to box as you have found. Getting a new machine is the best way. What do you see as a budget for a system. I would say an i7 rig would be a nice goal as most people have good luck running 5 toons on one. Once you set a price, then you decide if you can build one. If you have replaced RAM or a GPU, then you have the skills needed for the job