Normally in this situation I'd say go with the 13 because you just don't really need the better graphics card. But now that the 15" has the 9400M I'm going to suggest that size. I'm a student right now and I walk a couple miles a day with my 15" MBP and it never bothers me. The 13" would be slightly lighter, but I doubt I'd notice the size difference when carrying it. But I know for a fact I'd notice the difference when using it. I wouldn't even consider the 17".
I'd go for the base level 15", an external if you'd like, and some HDD/RAM upgrades, and/or some other accessories.
Monitor Recommendation: This Dell. Its really a fantastic monitor for a good price.
http://accessories.us.dell.com/sna/...etail.aspx?c=us&l=en&s=bsd&cs=04&sku=320-7825
A note about bootcamp: If you are only using 1 program I'd look into some of those virtualization programs. They have gotten pretty good now, I know people running programs like Solidworks no problem in them. Bootcamp is very annoying to deal with. I use a couple 3d modeling/rendering programs on XP and its just a hassle.
thanks a lot for your insight! and for the monitor recommendation... that price is much much easier to look at. but i notice (from your sig) that you're using a samsung monitor now?
re virtualization programs - i hate having to reboot into bootcamp. which one are you using?