I note that Crucial sells PC2-5300 RAM for the new iMac, (4GB at $320) as well as the more expensive PC2-6400 (4GB at $550). What's the difference, and is the more expensive version worth the extra cost? I had intended to buy PC2-5300 from OWC (4GB at $240). I'm sure OWC RAM is probably as reliable as Crucial's, but am wondering whether or not I ought to get the more expensive PC2-6400 memory...that is if there is any performance justification to do so.