The MacBookPro's will be a significant improvement over your Powerbook. If you want mobility, go for it, just don;t expect the power of a tower in your lap.
I almost always buy refurbished from Apple, look for it on the bottom left of their store page..
Refurbished MacBook Pro 2.33GHz Intel Core 2 Duo
15.4-inch widescreen display
2GB memory
120GB hard drive
6x SuperDrive (DVD+R DL/DVD±RW/CD-RW)
ATI Mobility Radeon X1600 with 256MB of GDDR3 memory
Built-in iSight camera
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? Save 25% off the original price
Original price: $2,499.00
Your price: $1,899.00
Thats a beefy laptop for a pretty good discount. They go quickly so just keep an eye out for the one you want...