what is the best disrto for a mac-ppc?

which ever one you prefer. there is no "best", all the time there is a thing called "free will" people will have their own "best" for their own reasons.

yellowdog is pretty good

Well,

I've heard good things (damned good things) about Yellow Dog.

But there's a Gentoo PPC release, as well as Debian, SuSE, and i think, Mandrake.

I know the SuSE PPC release is out of date (SuSE 7.3 I think).

Personally, I'd go with Yellow Dog unless you are feeling Hardcore, in which case I'd opt for giving Gentoo a go. Granted, this is all purely academic, because I've never used any of these on Mac. There are a number of reviews over at OSnews , so go and read a bit to see if anything strikes your fancy.

on a similar note: what is the best distro for PC? :P

Anyway i recommend you Gentoo, only cause i am gentoo advocate.
In general yellowdog will work too as will almost any other linux.

ok here let me fill you all in...
YellowDog - easy to install its allright
Gentoo - hard to install prolly the best
Debian - easier tto install seems slow yet still powerful (best server choice prolly)
Mandrake - blows
SuSE - never tried

Mandrake does NOT blow. Before stating non-sense, better put a pro-con list.

Ubuntu ppc iso page:
http://cdimage.ubuntu.com/ports/releases/hardy/release/

Allow me to give you my input (keep in mind this username is pretty old).

Congratulations on using a superior architecture. Sadly, you have to pay a penalty for it (PPC linux flash support is nonexistent, hardware manufacturers don't write drivers for architectures that aren't x86 and thus you have very little decent 3d acceleration). With this being said, you have a heck of a number cruncher and a great instruction set architecture.

My experiences have been thus far:
Yellowdog: Redhat package management with dated packages. A severely dated and license restricted distribution (it cannot even play mpeg out of the box)
Debian: I never really liked apt package management but it is very simple. Debian has gotten some bloat over the past few years especially and tends to run slow on my blue and white g3.
Ubuntu: Great support, albeit package management follows the debian model. Ubuntu dropped PPC support after dapper drake (6.06 or so). Expect packages to be old and if you want to have newer packages, you better use debian's repositories as the new ubuntu ones won't work (they don't officially have a PPC package repository because they stopped building the binaries).
Fedora Core: Very much like Yellowdog but still with icky RPM and some license restrictions. Will not install on an old world mac (unlike Yellowdog, which does support oldworld installations).
SuSE: You'll have license restrictions, Novell is known for this. Never really got into the microsoft loving crew, but the rep for Novell was actually a pretty nice guy.
Gentoo: A pain to build everything yourself but it ensures wide package management (mainly because you build it yourself anyway). Portage tends to be closer to bleeding edge than most distributions allow, which means if you're using the testing keyword, you'll have somewhat recent packages for PPC.

I personally use Gentoo on my macs with an overlay to portage known as sunrise (which allows for a really old reverse engineered version of flash). I can't honestly vouch that you'll get THAT much of a speed boast from the optimizations, but you inherently have less bloat this way and more control. I say gentoo ftw, even on an oldworld beige g3.