Rev. B Titanium PowerBooks go 1.2 GHz
If you happen to have one of those old TiBooks that run at 550 or 667 MHz, you can roughly double its processing power
by letting Sonnet Technologies replace the processor. A brand new shiny Freescale PPC 7457 with 512 KB of level 2 cache will give those older PowerBooks a new lease of life, although at 500 USD, you might want to think about getting an iBook 12" instead. That way, you not only get a brand new computer, but much more performance (thanks to bus speed and graphics card) but also USB-2. AND you can keep your old TiBook for other tasks (or sell it, of course).
I personally think that the market for such upgrades is rather small, although I can see some musicians lick their fingers, since those PBs can still run OS 9. Ah, nostalgy. ;)