First of all, GET A SSD!
I just got a new MBP w/ SSD (i7, 17", 8 gb ram) and it is amazing!
I got the 256gb SSD from apple and have had no issues, i found the price similar to doing it on my own.
I think you could get away with 128 gb if you are just going to run OSX and Windows, but no media.
Since mine is 17" it is more of a desktop replacement I usually keep it parked on my desk with a 4 TB Graid firewire external HD which is awesome, when I travel I just take my media on a WD firewire 840 gb (I think, maybe its 720 gb) external drive. If you are on an airplane you can just leave the external drive in your bag at your feet and run the firewire to your laptop, it really isn't a huge deal. I mostly use my media through Songbird and VLC player, if you are really into itunes you might have issues with having the library on both drives, not sure about this.