if that sucks on 6600's, but don't suck on a 6150, maybe the PCIe
host interface is penalizing you?
unfortunately my 6800 agp has died
so i can't run your test.
but to verify this theory, one should run the tests on any of this cards:
NV40 based 6800 AGPs
NV44A based 6200 AGPs.
note that 6600 AGP still has the PCIe interface and an PCIe <-> AGP bridge.
based upon all i know abut the PCIe interface, it would make sense that it
does penalize multiple small transfers.