Afaik, some N900 are/were actually able to reach that via overclocking.
Also afaik, not all Pandora software requires a 1GHz Pandora.

The Pandora's PND packages are basically compressed images of file systems with their own software environments.
The easiest way of trying if they work on the N900 would be to unpack a PND, store the data in some N900-compatible format (optified debs?).

The Pandora and the N900 should be very similar, performance-wise, however I see two drawbacks for the N900:
1. The Pandora's ecosystem has moved on, while the N900's hasn't. Pandoras are running state-of-the-art modern linux distributions while Maemo is mostly still stuck in 2008. Therefore it might actually be easier to try to port very old PNDs.
2. The N900 lacks proper gaming controls. I don't know if the N900's keyboard is able to rival the Pandora's, but the lack of a d-pad and nubs means that some Pandora software will be awkward, if not impossible to control.

All that being said, I believe it might be an interesting experiment to try to port PNDs to the N900 and I wouldn't be surprised if you'd find some old threads talking about exactly that, but I agree with the others, that for the actual sake of running PNDs beyond proof of concept, you should get a Pandora.

With the Pyra (hopefully) close to release, used Pandoras might become pretty affordable in the foreseeable future. Currently the price for used 1GHz Pandoras in good condition seems to be around 200 Euros.