I am trying to help someone add more RAM and I was wondering if a 256MB PC2100 266mhz chip would be able to replace a 128MB PC133 133mhz chip. I would do it and find out but the computer is 50 miles away. Any comments or suggestion help.

You're going to need to know the type of computer or the motherboard before you can decide what RAM can go in it. What determines what RAM can be upgraded to is the mobo, so you need that info. If it's a brand name pc, get the model and check the manufacturer's website for documents on upgradeability.

To answer your question, you can upgrade the REM only if theyre the same type. You cant use DDR RAM for DDR2 or SD for DDR. You can upgradr diff capacities and frequenxies of the same type of RAM