Don't do it. Even back in 2005, a 1.5 GHz 12" PowerBook couldn't play iTunes video files - how do I know? I had that machine and I tried it.
My 867 MHz first generation is still on my coffee table, plugged in and sleeping, but never used. It is by far my favorite machine, but it is beyond dated at this point. I've rebuilt it twice from magnesium frame and bottom case on up (replaced the top case/screen, optical drive, hard drive, bottom case) twice, and I don't think I'll ever let it go, but the deficiencies are hard to ignore.
- Slow. Not helped by software that's no longer optimized for PPC, if it's even available. Web browsing with anything Flash bogs down the machine. The 1.125 GB of RAM I have in there gets eaten by memory leakage within minutes. The 5400 RPM HDD that I have in there is immediately used as a scratch disk and that does nothing good for performance.
- Screen quality. If you've gotten used to the quality of modern Apple IPS displays, the dim, washed out TN display on the 12" will look awful in comparison - oddly enough, it still looks better than some ultra-budget modern laptops though.
- Fragile. I've replaced the bottom case due to too many dents, I've replaced the entire top case due to the latch breaking. Granted, I took this machine everywhere all the time, but it definitely should be babied. Anything you get now will likely not be in phenomenal shape either.
- Battery life. Brand new, I could pull nearly 6 hours out of the battery - no, not a modern, screen at half, wifi on 6 hours. This is a screen at min brightness, all apps except for TextEdit closed, wifi off, sound off situation. Normal use would see under 4 hours with a new battery. Now? 4 minutes on my good battery. Maybe. A new one (if you can find one) will be $130.
But it has the footprint of a sheet of paper, one of the best keyboards ever built, and I will never let mine go if I can help it.
My suggestion would be to find a good deal on an early gen MacBook Air - even a first gen SSD based model. I'm tempted to say you can get them for around $500 these days if you look, and while they were dogs in 2008-2009, they will be a magnitude faster and more liveable than a 12" PowerBook would be these days.
(though I secretly hope Apple will revive the 12" form factor with a new screen, no optical drive, and a modern processor... and keep everything else the same)