Advice please...

I currently have a 70-200 f/4L. I'm having trouble in low light situations because I only have a built in flash. If I make a decision soon, I can return my f/4 to B&H and upgrade to a f/2.8. Will this help my low light problem substantially or should I invest the extra $500 into a flash. I have considered keeping the f/4 and purchasing the 85mm f/a.8 for low light situations. Also, consider that I want to shoot some low light sports photography like football and basketball. Bottom line, what should I do to improve my low light photo and increase shutter speed?

Upgrade to the f/2.8 zoom? Get the new flash? Get the 85mm f/1.8?

If you're going to shoot basketball, get the 85mm f/1.8. It's definitely a keeper for gym sports. As for football, there aren't a lot of relatively inexpensive options. A focal length of 300 to 400mm is really better for football. Personally, I'm holding out for the Sigma 120-300 f/2.8. It's a heavy, expensive beast, but it gets the job done. I shot a college game with the f/4 version of the 70-200, but it was in the day time, and I was on the sidelines.

Thanks Liza