It's PowerBook, not Power Book, just so you know
The warranty doesn't get voided if you open it up to put extra RAM in. This is because you only have to open one panel to get to the RAM, you don't have to open the whole case. Another ingenious innovation from Apple.
You'll have to partition your hard drive as far as I know, because one section will have to be fat-32 and the other will have to be Apple. Once you do this, you can run it on both Macs and PCs. As far as I know. But I don't know much about external hard drives, sorry
Get the PowerBook 17".
Oh, Apple has stated again and again that there won't be a G5 upgrade until at least 2005. Considering Apple upgraded the PowerBooks considerably at the beginning of summer, I dont think they'll upgrade them to G5's until at least 2005. As for dual G4s in a PowerBook, no notebook ever (even Windows ones) has ever been dual-processor. Considering a single G4 chip runs hot as it is, I don't think Apple will ever fit 2 G4 chips in a PowerBook. Although that'd be wicked fast, I admit
Trust me, PowerBooks won't be upgraded until at least next year. Probably later than Q1.
Get the 17", it ROCKS! Good luck!