I am on a macbook pro that has relatively low RAM and video games sometimes will jump, so does a lot of graphic intensive programs, how do I make a scratch disk that any application will load some of its RAM into?
OS X does that already, it's called swap file. The issue is that RAM is trillion times faster than hard drive so everytime the data has to be written to HD instead, it'll cause lag.
To do it yourself is quite easy (at least it is on the generations with removable batteries, I have never taken a unibody apart). Pull the battery, take out two screws, pop out the old ram and pop in new ram and reassemble.
Any computer store should be able to do it for you if you don't feel comfortable.