Bill - to start with the last big first...I have no idea why anyone would advise staying with the Powerbooks at this point in time, if considering spending money on a "new" machine. Aside from the obvious - they're cheaper - they are not going to have much longevity IF you want to keep a current OS installed. OS 10.6 will not work on a non-Intel machine. While 10.5 versions may be available in parallel for a while, don't count on it for long. Apple will not continue to develop for the PowerPC.
But aside from that rather important (in my book) issue, there are many advantages to the MacBook Pros. Faster processors, bigger HDs, more RAM, stunning screens etc., etc. Downside...they will cost you more than an old Powerbook.
Back to the HD issue. I've always favored buying my HD and the enclosure separately - both because it is usually cheaper and I usually get a HD with a 5-year warranty. Most pre-assembled HDs have shorter warranties. Enclosures aren't usually what fails - HDs are.
Get a good enclosure with both Firewire and USB 2.0 (for forward compatibility) and a nice Seagate 7200 RPM SATA drive with a 5 year warranty.
A couple of options for enclosures:
http://www.newegg.com/Product/Product.aspx...N82E16817392018 (got a Vantec enclosure very similar to this for my aunt recently. Also have a Vantec 2.5" enclosure)
http://www.newegg.com/Product/Product.aspx...N82E16817182123 (I have a Rosewill enclosure - earlier version for ATA drives - and love it. VERY sturdy and matches my G5
)
http://www.newegg.com/Product/Product.aspx...N82E16822148288