the 256MB version has non state of the art ram in it, rather some 1+ year old ram on it, therefore has lowered clocks on the RAM, and to keep it from beating the 128 version too much, has a lowered core to boot.
your speeds seem accurate.
I have seen only passive heatsinks on the 256 version. if i were to own it, i would have copper ram sinks and a home made copper cpu heatsink and fan on it, and OC the crap out of it.
the 128MB versions have much higher core and clock speeds, so don't feel cheated... even though you were

...but they made powerstrip to fix that