I prefer
Darwinports over Fink. If a project isn't available through either Fink or Darwinports, you can attempt to build from source. There really is no such thing as x86 or PPC source code; if you download the source for say, LAME, it will build fine in OS X. Some apps can fail to build, but that happens across different Linux distributions occasionally just the same.
As a Linux to OS X switcher, there really weren't many GUI Linux apps I yearned for after switching; OS X generally does it better. The apps I do use under X11 are ones that I access so infrequently that they're not worth purchasing OS X equivalents of. The only app I use under X11 is PAN, because I'm too cheap to pay for Unison, even though its superior. CLI apps generally have binaries available or compile with ease.
As others have mentioned, VirtualPC and Bochs are pretty much the only emulators in town for x86 on PPC. If you need x86 emulation bad enough, I suggest you just break down and purchase VirtualPC. The last version (6.1) can be purchased cheaply enough on eBay, along with a copy of NT4, which blazes in VPC.
For PPC virtual machines you have a couple different options. In OS X, there's
SheepShaver, which can run older versions of MacOS, like 7 and 8 (maybe 9, I forget). It's still in beta stage, and last time I experimented with it, not incredibly stable.
If you decide to install Linux on your PowerBook,
Mac-on-Linux will let you run an OS X virtual machine inside Linux; since this is not emulation, the speed is close to native.
Generally, unless one has a specific reason, running Linux on a Mac just doesn't make much sense. OS X just rocks. However, when my machines begin showing their age, I'm glad to know they are Linux-ready.
