Thing is, I think the Dreamcast is closer hardware wise to the N900 than it is the PS2.

That said, the Dreamcast was able to emulate a PS1 (granted it only worked with a couple of games as it needed a LOT of optimisation per game) so it was a pretty able machine.

If the graphics chip is similar enough (the N900 one is basically based on the Dreamcast one) then perhaps that will help, but then probably not as most games will talk directly to the GPU on Dreamcast but all must pass through the API on the N900 - unless you plan to boot a custom Dreamcast OS negating the use of the phone functionality or any multitasking.