If you have the money to burn you should have a desktop first and portable second.
With a desktop you can always upgrade parts (limited for Mac), and can do other things that laptops can't do (or you just don't want to do it on your precious portable). But with PowerBooks, the limitation is far less than a lot of PC compatible notebooks.
I guess I don't have money to burn, so from time to time I upgrade parts for my PC, so it is a kickass unit as of now.
But for a portable unit I first purchased a Windows compatible notebook pc. I liked it and all, but saw the benefits of PB and got a 12" PB with superdrive (I should have skipped out on superdrive since I have a great burner on my desktop but....)
1. Get a desktop...any desktop
2. Portable unit (if you are on the go all the time, but since you are considering 17", you will not be hauling that thing all over the place)
However, if you are living in the dormitory, 17" PB may be easier to haul around. Luckily I have moved out of the dorms, but carring a 30LB desktop/20LBbackup desktop/1 flatpanel/19" CRT/2 laptops/ accessories is a pain in the ass as well as easy target for snatchers. Also, if stolen laptops are harder to use for parts and can be traced back (even though they are easier to take). If someone gave me a desktop, I could use so much of its parts, all that's left would be the outer case and some metal parts that I would have no use for.