In order to use all of the RAM you have available you'd need to upgrade to PostgreSQL 9.0 for 64-bit machines - in order to make this upgrade you'd need to backup your PostgreSQL databases, uninstall your existing PostgreSQL installation, install the new one, then restore from backup. Then if you run PostgreSQL Tuning from the Database Management menu, PT4 will set the config file to use the most of your RAM that it can.
Regarding the stats, you can use:
- Fold to F 2Bet / Fold to T 2Bet (these are folds when facing the first raise on the street in question - since 3bets and higher are rare, these should be very good substitutes for you)
- Fold to Raise After F Cbet / Fold to Raise after T CBet (these are folds to a raise after you've cbet on the street in question)
- Fold to PF 5Bet+
- Fold to F CBet in 3Bet+ Pot includes all 3bet pots and 4bet pots and 5bet pots etc so long as there is a cbet that is faced. Exactly 3bet pots are by far and away the most common (that have cbets anyway) so this should be pretty close for you here - you can run an All Players report in the My Reports page to compare these two values for most players and you'll see they're pretty similar (probably a few points smaller for some players and that's it).
The paired board and flush board stats don't have defaults, but hopefully this gets the most important ones you want.