I want to buy an iMac but probably not BTO, because if there's a problem with it, I want to be able to return it. I want to have at least 512 MB of RAM. My questions are: Should I just buy a 512 and then have 768 MB total? How much faster is the RAM if I buy two of the same 512's? And where's the best place to buy iMac G5 RAM?