if your sis' machine comes with just 128 megs, i doubt whether it IS a new one....
before you add memory into a computer, check for the following things:
whether the memory type is compatible- whether it is sd ram, ddram, again whether it is ddr1, ddr2 or ddr3.DONT EVER MIX THESE TYPES. THEY WONT MATCH. in case it is new, it will be a ddr2 or mostly a ddr3. so add only an identical module.
whether the motherboard can support it- some motherboards can support upto only 4gb,etc.
whether you mix and match the old and the new one.it is better if you remove the 128mb ram module while adding a new one. even though the system will recognize it, the machine performance will not be exactly the same, to some extent.also, a 128mb will be very tiny addition to a 2 or a 4gb module.
you can check this all with the help of a computer expert nearby.
hope this will help anyway.