Compare new (Mar 2011) 15" Macbook Pros
This question is so subjective because there is nothing specific being asked. Let me break it down for you.

How much does the move from 2.0 GHZ to 2.2 GHZ improve performance?
200 MHz is the obvious answer here. Improve performance, depends, chances are you probably won't notice any difference in that aspect.

How about the different graphics cards?

Obvious difference being the 768MB difference in memory in the cards. Subjective because who knows what your use is going to be, i can tell you for browsing/email/office type products you'll never see the difference. 

Are these improvements limited to certain types of applications?

What certain type of applications? Once again how can you answer this if you don't give anybody a specific area/application that you plan on using.

What do I need to keep in mind when comparing these specs?

Keep in mind in terms of what? Obviously you know the cost difference and some of the numerical differences, but what is it that you are actually asking here?

Sorry if this comes off harsh, but it's too vague and subjective right now to give you any type of concrete answer.