I'm not aware of the specific application for Mac, but it is possible to view your memory usage at any moment. Games and applications use just as much memory paused as they do when unpaused. So you can see the memory usage at peak load and see if it is very high.
Your system has virtual memory, physical memory, and paged memory. You want your paged memory to be less than your unused physical memory, and your total virtual memory to be less then your total physical memory (this is the same statistic counted in different ways). If that's the case, then Increasing the amount of physical memory will have no impact.
Few games are limited by memory. Most are limited by graphics cards, and some are limited by the CPU. So those are the parts that you should look to upgrade to have the biggest impact on performance. Also, for general purpose speedup, consider getting faster memory instead of more memory. There are a number of statistics for memory speed, but CAS latency is good one to judge by, and the others tend to follow it. Faster memory is not likely to be a significant speedup for most cases however, especially not graphics bound applications.
Ram is really only going to help with loading times and multitasking.
RAM won't help loading times. To decrease loading and startup times, you can upgrade your harddrive, possibly to a SSD.
