the old ones are good if theyre not going to be used for much of anything short of using the net or documents in office and stuff like that.
but if you plan to stress it a bit with photoshop, a game or two, or any sort of intensive work; id opt for the new alu-MacBook.
they offer better graphics performance which helps in overall system performance (as it isnt eating up your Memory like the previous built in onboard gpu); faster memory and FSB of course.
The display is much more accurate and higher quality than the previous, seeing them side by side, the older one was just unbearable to look at especially in bright (natural lights), the newer one while its concealed in glass has a higher density to lighting and making it easier to use in brightly lit places.
its a pretty solid machine and youll be able to run apps much more smoother with the specs it has compared to the previous line of macbooks.
goodluck!