15" or 17" choice, in my opinion is purely personal , what size screen do you want? I would never consider the 17" because for me it defeats the purpose of a laptop, it's far too big (for my uses). I have a 15" and it is perfect, the screen is ample, if you are doing lots of multi-windowed use, then a bigger screen might be nice, i plug an external monitor in at home when necessary.
The speakers are the best i have heard on a laptop, but again for high quality you can't beat a decent pair of external speakers, as the laptop one's lack depth (obv.)
There are a few games for the mac, look on Apple's website under games (i just got x-plane -it's great).
I would start witht the assumption that you can run no PC apps on the mac..! My experience of VPC is a slow one, at best. If you need to run PC apps, use a PC (note: some people may disagree with this).
The mac is v.easy to get used to once you dispel your windows way. It took me, from having never even seen a mac, 8 months. But, i didn't switch over night, i started gradually using it for mail, then WP and presentations and music and browsing etc, then i moved over for development.
Enjoy! I've never looked back.