Those are some good questions.
The Montgomery GI Bill provides you with 36 months of educational benefits, at whatever the current rate is for the bill. You have 10 years from the last time you left active duty to use up your benefits. If that time passes, any remaining months are lost. So, since you used up about 24 months, you should have 12 months of benefits left, which expire in 2013. To find out how many months are left specifically, call the VA’s toll-free GI Bill number, 1-888-442-4551.
Because your active duty service includes a significant amount of time after September 11, 2001, you are also eligible for the new Post 9/11 GI Bill. You could change over to the new bill, if you wish, but you may find it in your best interests to stick with the Montgomery GI Bill. You should check out the VA web-site and do the math (or talk to a VA education counselor) to find out which one suits you best. You would have as many months of the new GI Bill as you have remaining on the Montgomery GI Bill.
You can also use both. Due to the specific limitations of using different education benefits together, you can actually get some extra benefit out of the new GI Bill if you have completely used up the Montgomery GI Bill. If you have completely exhausted your MGIB benefits, you can still get another 12 months of the Post 9/11 GI Bill. But you have to completely use up the MGIB to get that benefit.
It’s always in your best interest to take a full load of courses (full-time), if you can. If you can’t, your benefits are reduced. However, that might be unavoidable, and it happens. But your college determines what is a full time course load (it’s usually 12 semester hours at a time). Speak to an educational counselor for your school about that.