I use a program called MemTurbo to monitor the amount of RAM is available.
By checking the amount of RAM available with no hash tables, then creating the table, then adding item(s) and watching the numbers in MemTurbo, I can determine how much memory is being used at each step.
The version of MemTurbo that I have is a purchased version, and allows me to configure the display to show Gig, Meg, k, or bytes. Note that the shareware version does not allow this customization.