I dual boot my Macbook with Windows 7 and OS X and I can run games on it like it's a PC. It's the most reliable computer I've ever had, and I work in the IT industry.
I do this too, it's great. The only extra expense is buying a copy of Windows. It really opens up the possibilities for gaming.
Agree with others about the video card. Definitely a big performance hit with that integrated video card...you may not think you need the extra performance but if you change your mind you want the flexibility (and my understanding is that the integrated video card in the MBP 13.3" is not something you can swap out or upgrade).
As far as the CPU, that will matter only if you do processor-intensive tasks - statistical analysis or something else. For most day-to-day tasks it is not going to be noticeable.
I'd also consider going to 16gb of RAM. RAM seems to be one thing - more than processor speed or HD space - that is still getting more valuable to have over time.
Also I think it's worth doing a quick search to see what upgraded models might be on the horizon. (The Mac Pro for example just got a huge re-design). You don't want to buy something and regret it when something superior comes out 3 months later.
http:/macrumors.com has some useful info about stuff like this.