I don't believe a Mac lost its soul once you put Linux on a PPC machine, you can still use "Mac On Linux" without completely ditching your OS X applications. Link:
Mac On Linux
Desktop GeForce 5200 has more texture/shader pipelines than the mobile version in the 12" PowerBook G4, same can be said of the 15/17" ATI Radeon Mobility 9600/9700 vs desktop versions. Linux in general is a crapshot as some video cards have multiple editions to support in the drivers, GeForce 5200 for example has 12 different versions(64-bit bus, 128-bit bus, slower clock speed, nVidia reference clock speed, OC-editions, GDDR2 vs GDDR3, single monitor vs twin, etc) and ATI is even worse they created more reference designs of the 9600/9700 than nVidia has ever done.
I'm not so sure Linux PPC development will last that much longer beyond Debian, Ubuntu no longer "officially" supports it beyond the web-only limited support, Fedora PPC is "dead"(after PS3 OtherOS was axed) and ever since Sony yanked "OtherOS" from the Playstation 3 Slim you have even fewer modern consumer PPC hardware.
Linux will give PowerPC a longer life, however the limitations of G3/G4 are starting to creep up vs G5 when it comes to processor/RAM headroom. YouTube via Linux is smoother than OS X but at the same time you're slowly hitting increased swapfile usage. (Xubuntu/Lubuntu is a dog on my 12" PB after a few hours)