My laptop (Toshiba Satellite M645) currently has 4 gb of 1066 ram in it. Would it be possible to buy 1333 ram and put it in? If it would work then would it be faster or should i just buy more 1066?
You can mix, but if you do it will run at the slower speed.
IIRC the M645 has two memory slots, which could both have 2gb each right now, so to upgrade you would buy a new pair -- check out your current configuration with CPU-Z (the SPD tab will tell you what is in each slot, if anything).