P4P800se and populating all memory slots?

All ram is PC3200.

I've tried using 2x512 basic Crucial modules with 2x512 Samsung
modules on the P4P800se board, and it wouldn't go past the bios menu
(I'd change settings, reboot and it would tell me overclocking failed
and take me back to the bios screen).

I then tried the 2x512 Samsung with 2x256 Mushkin Special 2-2-2, and
it would also take me to the bios menu, but I'd then have to reboot
twice to get it to boot XP. And when it did work, it ran the memory
at 266mhz.

Can you run four modules on this board? Doesn't seem like it.