" My computer has 1 DIMM of DDR/PC2100 ram (128 MB of it). Can I take any
brand of DDR/PC2100 ram and put it into the other slot? Must the other ram
be 128 MB, or can I combine a 128 stick and 256 stick? Can I put in a PC2100
stick and PC2700 stick, or are they different sizes? "
If your system came with PC2100 DDR-RAM, then your motherboard and CPU ar
operating at DDR266. PC2700 DDR-RAM operates at DDR333, and you can't mix
these speeds.
You may be able to mix RAM sizes, but it isn't always recommended. If your
128MB DDR-RAM module supports ECC (Error Check and Correction), then you
will be less likely to have problems in adding a 256MB DDR-RAM ECC module,
but it still isn't certain that it will be perfectly reliable. Even mixing
brands of the same spec RAM can bring up reliablity issues.
You would do well to find out what motherboard you have and what it supports
in terms of RAM. Some relatively modern DDR266 motherboards have a total
RAM limit of only 512MB that they can support, so check this before doing
anything.
If I was in your position, then I would opt for two PC2100 ECC modules of
the same spec from the same manufacturer, either 2 x 128MB or 2 x 256MB. I
would then sell my 128MB module on eBay with a full description and photo,
as it is guaranteed to sell for close to what you pay for it new.