As far as I know, only PPro CS5 has MPE, and further, it has hardware MPE with certain nVidia cards
For a list of supported nVidia CUDA cards go to the FAQ
For others, the nVidia Hack http://forums.adobe.com/thread/629557?tstart=0
PrE 8 does not have MPE (the Mercury Playback Engine). As John T. mentions, only PrPro CS5 does. Whether PrE 9 (due out in some months?) will, is not known. No rumors have been passed about - yet.
I kind of doubt that it will, as it is a fairly expensive feature, from a coding standpoint. Also, it is limited to but a few high-end video cards, and this would preclude its usage by the majority of PrE users.
It is only a guess, but I'd say to not look for MPE in PrE for at least one more version, when there are more, and lower-end cards, that fully support. Now, some aspects of it (those not exclusively limited by the upper-level nVidia cards) might find their way into PrE 9, but I highly doubt it.
For CUDA/MPE support, you will need one of the select nVidia cards and PrPro CS5.
Good luck, and remember that these are only guesses.