well, it'd have been better with two dimms for it to take advantage of DualChannel memory. It's unusual for a PC to be equiped with only 1 Dimm when it's capable of DualChannel.
Perhaps that is also the problem. This means your only getting the memory throughput of 533MHz instead of a possible 800MHz (or if it was run in dual channel 533MHz, 1066MHz). And ontop of that, because it's nowhere near in sync with the FSB, it's more inefficient.
I dunno what to recommend. Just to see if it has an effect, changed the Memory FSB down to 400MHz. I'm not expecting it to do much, but you never know.