Tracking down performance issue

I'm running 7.4.2 on an older Linux box (450MHzAMD K-6-III, 450M RAM)
running kernel 2.6.5. My client is a Java/JDBC program on Windows.

I'm having trouble seeing where the bottleneck in my performance is.
The client uses about 30% CPU. The server uses 70% CPU plus 1.5% I/O
wait. The I/O wait is very low because I'm doing a PK index scan where
the index and data are on different disks and the table is clustered on
the PK index. The network is 100Mb, and it's at 7% of capacity.

I tried making the client operate on two threads on two database
connections. That bumped the server utilization to 80% and barely
changed the I/O wait. The throughput decreased by a third.

The only thing I can think of is memory bandwidth. Does anyone have
tips on how I can investigate more?