Thinking about getting a new mac, but not sure when or what!

A little background:

I currently own a PowerBook G4 (processor: 1.67 GHz PPC, memory: 1 GB DDR SDRAM). I bought it in August 2005 when I started college. It lasted okay for the 4 years of college, though with problems of course, most of which I'm still experience. None of these are HUGE problems to ditch this altogether, especially seeing as how much new Macs cost.

HOWEVER, I've been thinking about getting a new mac. Needs:
- With my long commute to work on crowded public transportation, I'd like to not carry my laptop on a daily basis.
- The new mac will mostly just stay home. I work in a science lab, and would prefer to not have chemicals all over my new mac So really, I'll just need this new mac for music, movie-watching, internet usage, etc. I'm not big on gaming and I won't need to run many fancy programs.

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Now the questions:

Firstly, I'm debating between a MacBook or a MacBook Pro. For someone NOT very familiar with computer language (for instance, I don't actually know what PPC vs. Intel means... or what DDR SDRAM means), and someone not using the computer for intense programming work, what are the differences b/w a MB and a MB Pro from a layman's/consumer's point of view?

(All I know is that the former is obviously cheaper. But the keyboard looks like it might get obviously-dirty really quickly)

Secondly - is the extra few hundred dollars worth spending for 13" vs. 15"? I currently have a 15" and it's great. So I'm not sure if downsizing to 13" (if anything, for financial reasons, obviously) will make a big difference, especially if this new mac ends up being my primary laptop eventually (in the case that my current one decides to die completely). Any experiences?

Thirdly, with the launch of the new iPad soon, and some rumblings about a new line of MacBook Pro's coming out (or no? maybe I misread), is it better to wait? (From the point of view of new features and/or price changes if any.)

Lastly -- in general, what are the advantages of buying online vs. at a store??? I live in Boston, MA, and have a big apple store fairly near me. I know in-store purchases make you pay tax, versus none for online purchases. However, since I work full-time, I think that might make it difficult to receive the package... any other considerations?