Greg,
I've shot Agility Trials in a covered arena and used a 70-200 F2.8 with good results. Anything slower and there just isn't enough light to stop the action. Nobody allows strobes at an Agility Trial I've shot.

Sorry, but if this is what you're going to shoot, you need to spring for the F2.8 zoom IMHO. Look at Sigma, Tamron and other 3rd party lenses if you can't go for the Nikon or look for used or refurbished at Adorama, KEH or one of the other reputable retailers.

Jim