Memory confusion

[Hot Tub]

I have a Dell Dimension that has one stick of NEC 256 NON ECC Rambus memory in it. There is a filler stick in the other slot. I would like to upgrade to 512 by adding another 256 stick. Do I have to buy a "matched pair" or can I just buy one 256mb stick from another manufacturer and put it in along side the NEC memory that I already have. I have been looking around for a duplicate stick of NEC memory but have been unable to find exactly the same thing. What would be the least painless way of getting my memory up to 512? :(

[bachviet]

Another reason why I still stick with DDR RAM.

[Hot Tub]

I have a Dell Dimension XPS B1000r. I checked the Crucial web page and it says that they do not have any memory for my model. It's starting to look like the computer is going to stay with 256MB unless I can figure out what to do to upgrade it without having to sell the house to pay for it.