I am updating an older PC and want to increase ram. Currently the machine has 512MB on a pair of 256 "Non-parity DDR PC3200" modules. Machine has 2 more empty slots. I would like to add at least 1gig on top of this -- can I just buy one 1gig module, or should I get 2x512? Is 1.5 gigs an "odd" amount? Finally, is crucial.com a decent place to order ram?