To be honest, Rambus is way too expensive, you would in the long run be best off selling what you have and looking into a new system. Not only will finding compatible Ram be difficult, but the cost will far out way the worth to you.
For Rambus (similar to old style Simm) They must be installed in matching pairs. You can have a pair of 256mb and a pair of 64mb, but you need to at least match the specifications on those.
By the way, it has nothing to do with HP, its just the pitiful design that Intel started their P4 line off with.
If you are finding this Ram very cheap, I would be very careful about it, as it they might have compatibility issues or not be functional.