Here is what you need to know and based on that,  U can choose what is really feasible for you. You can go to either community college or join Kaplan. In both cases, you are maintaing F-1 status. My rommie join the community college for a year, and I went to Kaplan. However, you have to convince the International advisor from community college to get admission. And, usually they issue I-20 for a year. In other hand, Kaplan I-20 depends on what course you will be registered. For GMAT, its a 3 month, and possibly extension of another 3 months. So a maximum of 6 months. In case of GRE, i think it is even less. Don't know about MCAT or others.

This is my advice for you. Since you want to stay from Jan to August , Join community college. GMAT @ kaplan costs you about $2200, and also you have to spend around 30 hours a week, no matter what. Now if you are working, things gets tuff here. But if you are serious about ur master programme and wanna get into best college with scholarship, this helps. while if u just want to maintain status, community is better . On the other hand, by joining some community college, ur r just wasting some money and time for classes that you will never need, unless they are required for masters or say in best case you wanna know about things - like may be a photography or design or even computers. 

Also, you have to know that Kaplan doesn't offer GMAT or GRE or even other class in certain schedule. They start those classes when they have enough ppl for the new session. U should also know that you have (unless things have changed recently) 60 days from the day ur OPT expires to issue a new I-20 and on top of that another 90 days to start a class. SO let's say ur OPT expires in 01/15/2009. Then by 03/15/2009 u should have ur I-20 and ur records in sevies. and Ur class start date should be before 06/15/2009. SO basically even U can issue I-20 early enough and join class in Summer -1 ( which usually starts from  end of May in southwest). But now you have to take 12 credit hours.

Hope all these helps.