i got a Dell E521... the ram installed is 2 1 gigs of pc 5300. im not sure if its 533mhz, or 667. my question is, does it matter, can i mix mhz? see, i want to add 2 more 1 gig sticks, and i found a good deal on some pc 5400 667 mhz, but im not sure if i can use them. id even get some pc 6400 if i was allowed. also, i went to dell web site and looked at the ram upgrade for the E521, and it said it supports PC2-6400 / 800 MHz??? Crucial and kingston but say it doesnt support 800. so if any one out there can give me a few dos and donts, that would be great. thank you.