Hi asultana_80,
Many Business School accept the GMAT or the GRE as part of the application process, so you should check with the School to see what your options are. If you have an option to take either Exam, then you might want to take whichever is 'easier' for you (given the limited timeframe you're working under).
Your GMAT score goal is modest, but we don't know your current "level" of ability yet. It's important to get a sense of your strengths and weaknesses early on in the process (so that you can define them and work to improve them), so it would be a good idea to take a FULL-LENGTH practice CAT test sometime soon. You can download 2 for free from
www.mba.com. A FULL CAT takes about 4 hours to complete, so make sure that you've set aside enough time to do so in one sitting. You will likely find certain parts of it to be challenging, and you might have pacing problems, but all of that is okay for now. You might also want to do a little bit of studying before you take the CAT, so that you can familiarize yourself with the question types.
Once you've decided on which Test you'll take and have a practice CAT score to review, you should post back here.
GMAT assassins aren't born, they're made,
Rich