I have 3 sticks of ram/memory and I want to know know which combo will be fastest...

2 sticks PC2700 128mb
OR
1 stick of PC2100 256mb

or mix the two? then which one goes where...

 

It all depends on if your processor is set to run at the speed of 2700 or 2100. If your processor runs stock at the 2100 speed, then your 2700 will only run at 2100 anyways. Also, you should check it your motherboard is DDR compatible, if it is, then run 2x128, if it isn't, run either. You can mix the two, but I don't recommend it, it's always better to have the same memory to have more stability and besides, all memory you have in there will only run at the speed of the slowest stick (ie. 2700 will only run @ 2100 if you have a 2100 mixed in there).

 

If you have DDR RAM, which it seems like you do, pairs need to be installed in the same colored slots:



Various different motherboards have different colors. If you only have two chips, use the color closest to the processor. In that pic, it would be the blue ones.

As far as determining which would be faster... I have a hunch that running the 3 chips at a slightly lower frequency would yield better results. Just because a FSB is capable of running a certain frequency doesn't mean it always does. Apps can always benefit from more RAM. If you can't decide, try it both ways and see which way seems faster to you.