performance of PostgreSQL on 64 bit MAC OS X G5!

Hi,

We have been running PostgreSQL 7.3.4 on 64 bit MAC OS X G5 dual
processors with 8GB of RAM  for a while.
Lately, we realized that consistently only about 4GB of RAM is used
even when CPUs have maxed out
for postgtres processes and pageouts starts to happen. Here is a
portion of the output from TOP:

MemRegions: num =  3761, resident = 41.5M + 7.61M private,  376M shared
PhysMem:   322M wired, 1.83G active, 1.41G inactive, 3.56G used, 4.44G
free
VM: 14.0G + 69.9M   277034(0) pageins, 1461(0) pageouts

Is it because PostgreSQL 7.3.4 can't take advantage of the 64 bit
hardware or is it something else?

Thanks a lot!

Qing