hmm... just so you don't get disappointed again, merom doesn't give much more juice than yonah. period. not at least until santa rosa will you even see anything substantial.
of course, that is unless you are among the 1% of people who really use your current mac to 99% most of the times (ibook... IBOOK? YOU USE IBOOK AND YOU THINK YOU'LL NOTICE THE DIFFERENCE? no offend to you personally, but the stuff you can run on ibook will not make a difference on mbp yonah and mbp merom)
also, if you are really among the 1% of ppl aforementioned, you would be using mac pro (or at least powermac g5, quad) instead of imac
again, no offend, but i really cannot stand every body to think that yonah is not gonna be enough for them for some reason, or that merom is gonna do something for them that yonah cannot... c'mon people, wake up, yonah 2.16 is an overkill for most of people already.
(yup i just said that)