Hello. I'm debating between getting a 70-200 f/4 or a 200 f/2.8. I will probably use this for some everyday use, but really want it to photograph indoor sports, mainly basketball. I was wondering if the f/4 would be usable in gym coditions...I use a 40D and don't mind bumping up the ISO. I need my budget to stay under 600 dollars and don't mind buying used. Thanks in advance for all your help.