There was a 900MHz G3 at one point, I believe, but AFAIK this is the only Pismo upgrades still in production. I confirmed by email with Wegener just last week that they are still shipping.
If you are planning to run OS X, the G4 would give you some improvement. If you're sticking with 9, and you have the 500MHz CPU, you could try overclocking it :rambo: