You should probally try to run your cpu and ram 1:1
He's got a HP oem board, those things are locked up pretty tight in the BIOS. If your cpu is running at 800mhz fsb, this means that it is 'quad-pumping' a 200mhz system bus (4x200). The ram, being Double Data Rate (DDR2), doubles this bus to run at 400mhz. Even if you have 533mhz ram, the system will still only use it at 400mhz unless you overclock the system bus. But I'm guessing that there are no adjustments in BIOS, right?
I would go for the 533 (or higher, there wasn't much difference in cost the last time I looked) ram so you can use it on your next, and hopefully clockable, system:)