I firmly believe if a computer can do what you want it to do then it can never be obsolete.. for most of my work which is really, "everyday" basics.. with some exceptions my G4 PB 1.67 Hi-res w/DDR2 memory can do most of everything I throw at it.. and with new codecs I found using mplayer OS X Extended I can now play 240- up to 720p on my G4 1.67 - not bad at all..
And for 150 I installed a 128GB PATA SSD drive and it breathed new life into my PB G4. While yes, I do have my 2010 Mac Pro 6-core - I only use this for extreme high definition video, dvd ripping, handbrake etc.. but 70 percent of what I do is handled by the PB G4 and it does it quite nicely..
I am contemplating getting a G5 Quad again as I had this machine 2 years back and thanks to it, I got into my Intel transition.. I believe both PPC and Intel serve unique purposes.. Leopard is still a nice OS, but so is Linux and although Apple abandoned the PPC and Lion no longer supports Rosetta, I am sure Linux will keep the PPC macs going for a long time to come.
Intel and PowerPC - two great reasons why Macs are cool!