Just for posting this topic, I've increased the PM capacity for all the member groups, except PH Newbie. Like CouchPotato mentioned above, you get more space as you reach different status.
The reason we put a cap on the number of PM is because PH only has a limited amount of space on the server. We also want to keep the database clean so that it is easier to move and restore in the event something bad happens.
Here is the breakdown for each group:
PH Newbie = 100
PH Regular = 300
Jr. Poster = 500
Sr. Poster = 750
Elite Poster = 1000
Note: It is unwise to store any private info in PM or online. Print out or save a copy to your computer.