White MacBook late 2006 RAM upgrade question

So, as I understand it, my macbook can use up to 2gb of ram (it came with 512 or whatever). I went to the apple store and the girl showed me a link of what to order... I ordered it without really looking to hard (i know very little about computers and i trusted her...big mistake i think) and it just arrived and it is 2gb but it is a single module or whatever.

Please let me know what would happen if:

1) I put the 2gb into one slot and left the other empty.

2) I put the 2gb into one slot and left one of the old 256 ones in the other slot.

Will it make such a difference that I should definitely order 2, 1gb modules?

Thanks in advance!