Just an observation:
According to the HP specifications for the model, it uses the PM965 north bridge, which is the same as in my DV9700.
My DV9700 currently has two Corsair 4 GB DDR2 800 MHz modules installed, running at a speed of 667 MHz. Additionally, a quick glance around my usual haunts on the net says that the PM965/GM965 is in fact limited to 667 MHz for memory speed.
While ANY spec listing is potentially wrong, enough of them in respectable places tends to convince me short of proving them wrong myself.
I DID find some forum posts from people stating they had 800 MHz and even 1066 MHz modules running in their system, I would tend to think that they are simply able to boot the system with such modules in place. That doesn't mean they're actually getting the speed, especially as the PM965 doesn't even support 1066 MHz FSB for the CPU (or else I would've been ALL over a better chip than the T9300).
As unfortunate as it seems, 667 MHz does seem to be the limit. Just wanted to toss some further evidence on the pile since I have a similarly specced system.
O Shadow Fox O