DDR RAM newbie here
 

I have 128mb DDR PC 2100. My question is what kind of ram can I buy to upgrade. Does it have to be 2100 (266) or can it also be 2700? and 3200? Will an 266 and 400Mhz be compatible. BTW what should I look for when buying it. What kind of CAS, MS, etc.

depends on your motherboard ... but most of the time if the board dosn't support the faster buss speed it will run at the slower speed

im not really a hardware guy but why don't you check on motherboard manual if it allows other RAM speed.

what does ECC or NON-ECC mean

ECC is "error correction control" which checks the data in the memory for errors, this slows down the transfer of data but only by about 1% from NON-ECC. Desktop PC memory is almost always non-ecc, ecc is used for servers mainly.