I suggest going for the rebirth Edition. Currently the 1 GHz model will only give you slight performance gains when running certain android games, n64 emulation or emulating x86 PC Hardware (qemu, dosbox). But more in a sense of "it'll run a little smoother (maybe with more details)" rather than "it will only run with the 1 Ghz model.
You could also go with the 256MB unit, if you don't care about android (512 MB have a newer revision of the SGX, that has some bugfixes) or Java based games/apps (the more memory the merrier) at all.
I would not recommend to put your expectations high on n64 emulation - there are some titles running rather well, but afaik only Mario 64 runs more or less flawless - read
here if you want more info about how n64 emulation performs on 1 GHz Models.
Last but not least, do you know
EDs comparison table ?