Guild Wars is 3D and the Mac client is in Beta, for that you will require the 15" as the 13" has no discrete graphics.
No specifications on what the software will run on, but 3D games often require the most powerful video card graphics you can get and the option to upgrade it as the game gets improvments. You can't upgrade the video card in laptops.
I really don't advise any Mac for 3D gaming, a Windows 7 tower is much MUCH better suited to task and you can upgrade the video cards from plenty of cost effective options.
The MacPro is due to be axed it appears as Apple hasn't updated it for ages, the only video card upgrades (the few there are) cost 3x more and are 2x less in performance than compatible Windows PC versions.
Perhaps your better off on a 13" MBP with SIMS there (it's mainly 2D graphics) and then a Windows 7 64 bit gaming tower with 4GB of RAM expandable and the option to upgrade the video card every 6 months so you don't fall behind for the heavy duty games.
If your budget is tight, a console works ok too.
Scroll down until you find a Mac, you'll see what I mean shortly.
http://www.cbscores.com/index.php?sort=ogl&order=desc
http://www.videocardbenchmark.net/
Also you need to keep your Windows 7 skills up, as 95% of all businesses use it, so you need to get a job one day. đ