My motherbaord suports up to DDR2 533, but can I plug in a stick of DDR2 667 and it'll just run at 533? I'm thinking of upgrading the motherboard in the future so I'd rather get 667
Yes, good thinking - I would never buy PC2-4200!
PC2-5400 is still inexpensive, and will work fine.
Ideally you would pickup PC2-6400 (800MHz) if you were thinking about the future.
It will also work fine @ 533MHz - really fine in fact :^)
My Corsair PC2-6400 has special SPD settings, that kick in below 270MHz (or ~540DDR) to apply much tighter memory timings @ DDR533 speed.
Regards
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