Hardware opinions wanted

I wanted to solicit some opinions on architecture and performance from
you guys.

I am torn right now between these two systems to replace my aging DB server:

4 x 2.2 GHz Opteron
8GB RAM
Ultra320 15kRPM RAID5 with 128MB cache

and

2-way 1.2GHz POWER4+ IBM pSeries 615
8GB RAM
Ultra320 15kRPM RAID5 with 64MB cache

I plan on serving ~80GB of pgsql database on this machine.  The current
machine handles around 1.5 million queries per day.

I am having some trouble finding direct comparisons between the two
architectures.  The OS will most likely be Linux ( I'm hedging on AIX
for now ).  The pSeries has 8MB cache per CPU card ( 2 CPU on a card )
while the Opteron has 1MB for each processor.  I know the POWER4+ is a
very fast machine but I wonder if having more processors in the Opteron
system would beat it for database serving? FWIW, they are very close in
price.

Ignoring the fault-tolerance features of the pSeries, which one would
you pick for performance?

Thanks,
Dan