PocketNester is the best NES emulator for the Treo. Plays games perfectly, the only nitpick I have is that it doesn't go to true fullscreen. Someone savvy enough might be able to edit the PocketNester Registry entries or hack the .exe to fix this though. Hint, hint.
Now for SNES Emulation. PocketSNES is okaaayy, but slow. And it hasn't been updated since 2003. And it is not fullscreen, in fact worse as the aspect ratio isn't correct so it distorts the image.
This emulator looks very promising. Its super fast:
Snes9xj4u
Setup: You should download the first ARM link (version b1k). Unzip, copy to anywhere (I popped the files into my start menu). Run once, then shut down (file -> bottom option). Open up your favorite registry editor (eg PHM RegEdit) and go to local machine\software\ce44u\snes9xj4u. Change Language from 0 to 1. Save and exit, start up Snes9xj4u. Should be in English now.
While your in Registry, you should change Autoskip to be set to 1.
I'm still playing around with it. Unfortunately it suffers from similar full screen issues & lower compatibility.