Hi, I have a similar machine P4/1.7 and it takes RAMBUS PC800 RDRAM.

You need to insert the memory in matched pairs and you need continuity strips in any unused slots.

My machine has 2 x 128 and 2 x 256 strips.

So, you will need to take out 2 of your 64's

I am surprised that you are offered anything other than matched pairs. Perhaps you should check that out with the shop.

If you are offered single RIMMS the implication is that something nasty has happened to its companion

EDIT: You could take a look in your computer or motherboard manual................there is usually a RAM compatibility chart/table. Or try a memory supplier's "finder" software.

There are a very few PCs that will support RAMBUS memory not being inserted in pairs. Others have a specific insertion sequence.

Given that you have matched strips in there already, I would say it is of the type that requires matching, after all you would not put in the second pair of 64's if a 128 would do the job?