The difference about using a memory cap in a WPC machine is that in addition to preserving the memory it also has to support the real time clock, so the power draw is greater. I've heard of people getting 2+ weeks with a memory cap and WPC which is probably ok for a collector who plays their game sporadically.
Unless your game has a midnight madness mode or you are so fastidious that your machine has to have the correct date and time, you're better off putting in a NVRAM solution IMHO.
viperrwk