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:

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The Powerlogix 900Mhz G3 was a dog - it overheated (considering it relied on the existing 5g copper shim to cool it), and I don't think many of them lasted long.

I've tried overclocking Pismo G3 CPUs in the past, and haven't gotten far - the "ceramic" G3s used in these are really only good for 500Mhz. The 400Mhz gets 450Mhz at best (I've done about three), and the 500Mhz doesn't go any higher (two attempted). It could also be the cache crapping out, but I'm not so sure.

JB