any homemade processor card will run into the same problem which sonnet had when they tried to sell dual 1,8 and 2,0 GHz; the shit is getting too hot.
a quicksilver with dual 1,6 or the MDD dual 1,42 are pretty fast machines, SATA II is possible there, and if you add a protools farm and a 30" monitor you can work with that for the next decade.
Yeah, 3GHz is absolutely out of the question without, like, dry ice chucked in with it. But I could see a 1.4 upgrade, or even single 1.8, with proper cooling. An MDD or Quicksilver is the obvious choice, but it might not be as available as anything earlier given their reputation as the be-all end-all of the G4 series and thus the most versatile from a retro computing standpoint; looking on the 'Bay, I see sellers not shy about charging $260 and up for one, where I bought my Sawtooth for $36, and didn't have to pay taxes or shipping, enough to buy a GPU, new processor, hard drive, and keyboard and still have paid far less than for just a computer, with room to spare for getting a full set of RAM. Part of the reason is because of the higher demand, and part of it is because MDDs are more often sold by collectors/old Mac people rather than just anyone.
If there's any way to get the older machines on at least the level of a QS, it's worth at least running the numbers on, even if nothing ultimately comes of it.
Plus, this is just my seemingly entirely unique opinion, but I kind of think anything after the Graphite is kind of ugly. I realize that it being the better computer is all that matters, but it's still there.