Slow memory throughput

I recently assembled a system for a family member and everything appears to
be working flawlessly... except that the memory throughput is very slow
compared to my (comparable) Pentium 4 system.

The new system has an ASROCK K7S41GX
(http://www.asrockamerica.com/Products/K7S41GX.htm), an Athlon 2400+ CPU,
and 512 Mushkin Blue PC2100 memory (tweaked to highest CAS etc. timings
allowed in BIOS). The CPU is the 266MHz CPU FSB version, and I've verified
that the memory is running at 133(266) MHz. I even cranked it up to 166(333)
because the Mushkin memory can handle it without barfing, but even that
didn't help much.

The system's PCMark2002 memory benchmark score is only about 2600, whereas
my Pentium 4 system with a 2.4-GHz P4 at 400 MHz FSB and 512MB PC2100 scores
about 5300. That's disappointing, because I thought that the two systems
would at least be in the same solar system WRT performance.

Is the SiS chipset to blame? Is there any other setting that I should try
in the BIOS. Any help would be appreciated.