Most Pandoras can overclock to 800Mhz with no problem, which seems to be enough for most games. There's a tool built in that will allow you to overclock to as high as 900Mhz, but most Pandoras can't go that fast: if you overclock beyond its limit, the system will freeze and you will need to reset; no other problems have been reported.
It's also possible to increase the voltage to get as high as 1Ghz and while there is a hidden tool that makes it easy with some safety precautions built in, OPT don't officially condone increasing the voltage to get more speed because it can wear out the CPU faster and drains the battery like nobodies business.
edit: oh, Dreamcast, there was a techdemo which showed a lot of promise, but the developers have been waiting on their Pandora for so they may have lost interest in the project. Someone else mentioned taking it up. I wouldn't be surprised if there is a Dreamcast emulator eventually, it'll just take a little longer.