high %system time

We recently upgraded from php 4.3.10 to 5.1.2, and the %system time has
skyrocketed:

Cpu(s): 42.8% us, 43.6% sy,  0.0% ni, 11.3% id,  2.2% wa,  0.2% hi,  0.0% si
Mem:   8312844k total,  7566168k used,   746676k free,    22356k buffers
Swap:  2040244k total,      520k used,  2039724k free,  6920384k cached

it used to be around 5% to 10%.  The server is a quad-xeon dell pe 6650
running CentOS 4.2 with 8G of RAM running pg 8.1.1.

How can I determine what is causing such high system load?  it seems to
have immediately jumped with the php upgrade.

thanks,
--
-Jacob