If the mobo can run the memory at 667, then it'll be faster than 533. If not, it'll run it at 533 or less and there's no difference between them. (Note: Timings may be different, but the difference in performance is virtually unnoticeable.)
If the mobo is dual channel, then you will want to run 2 matched sticks. If it isn't dual channel, either (2x512 or 1x512/1x1gig) will perform about the same (of course you'll have the extra memory if needed in the 2nd case).
If it is dual channel, and you really need more than 1gig, get 2x1gig. While 1.5g will work on a dual channel mobo, it won't run in dual channel which will impact performance noticeably.