I will attempt to answer all of your questions and concerns as well as I can.
Lets go over your background section you have listed. First, ANY Apple computer is designed with power users in mind. The OS is already 64 bit, something that windows users are just now getting caught up on. Your memory will determine how many programs you can have open at once, that is true on all computers, Windows or Mac. You will find however that OS X handles memory better than Windows does. My Mac side of my MacBook is screeming fast with 2Gigs of RAM, but my XP side is much slower. I have read many times that above 1 gig of memory, XP doesnt gain any additional speed, and in fact is probably slower.
I would imagine a Mac Laptop would be good for your field, but since I am not an Accountant, I cannot be sure of this. It will run any programs that a Windows computer will run. In fact, if you really want, you can load windows onto your mac and run both! I have this setup, but I use windows less and less the more I use the Mac side. The Mac OS is SOOOO much better than Windows its hard to believe Microsoft is leading the market. OS X is far from the Operating System you may have used in school before. It is phenominally better than previous versions of Mac OS. I doubt you will have any issues whatsoever transitioning, other than having programs that wont be compatible with the Mac side. Load XP with Bootcamp and you can use ALL of your programs, including Office for Windows. Macs are well built, and despite what you may see here, are some of the best computers around. Apple has consistantly had a higher rating of consumer satisfaction than most other manufacturers and that is a FACT. In your case I would probably lean towards a PRO. You get the upgraded screen, graphics options...but thats about it. The rest is the same. Procs, memory, hard drives, disc drives, airport, bluetooth...oh wait, the backlit keyboard...which is pretty cool, but I dont do much work in the dark.
Ideal configuration is to get it stock. Apple charges an arm, a leg, and an ear for upgrades. Buy the RAM somewhere else and install it yourself. It is easy and the users guide of your new mac will have instructions on how to replace the memory. Good memory makers include: Crucial, Micron, Patriot, OWC...etc. I have 2Gigs of Patriot in my MacBook and it works flawlessly. They have GREAT reviews, and are CHEAP. I got my 2 sticks for $149...Apple charges $600 for 2 gigs of RAM. Ouch! Bigger hard drive?...same thing...Just get the biggest one you can and install yourself. Really easy, just slide old one out, and new one in, install discs and youre all set. Obviously you want the best graphics card you can get, unless you get the MacBook and not the Pro, and you are stuck with integrated graphics, which isnt bad, but gaming is not so good. Office is available for Macs...Although I would probably just run XP with bootcamp and install Office on the windows side. I dont like any Microsoft stuff molesting my nice OS X softwares.
Thats all I have...seems like everything I think. Wow, my hands hurt now.
GET THE MAC. YOULL LOVE IT. AND SOON YOU WILL BE ONE OF US!