Slower MySQL inserts for AMD64/Opteron?

[ray at redshift.com]

I did some benchmarks recently on a single 2.4Ghz Xeon against a Dual 246
Opteron and noticed when it came to MySQL inserts, the single Xeon was about 20%
faster on inserts (and it actually had a slower hard drive, IDE vs SATA).  The
interesting thing is that the Dual Opterons was twice as fast retrieving the
data using selects.

Does anyone have any ideas as to why the single Xeon would beat it on inserts?
I've seen other benchmarks floating around (e.g. Tom's hardware guide) that
seems to mirror the results I found.  I also spoke to AMD about it and they only
suggestion was to try running i386 on the AMD, as opposed to AMD64.  They also
mentioned something about perhaps compiler switches, but so far I haven't heard
back, nor have I had a chance to test further.

The Xeon was running 5.3/i386.  I believe the AMD was running 5.4/AMD64.

If anyone has any ideas as to what might be the cause, I would be interested in
hearing.  I'm getting ready to load i386 on the AMD (using a spare drive) just
to see if this makes a difference.  Am I over looking something that is required
to unlock the power of the AMD?  The selects being twice as fast was great, but
then again the AMD has two CPU's also.

Thanks!

Ray