well, the faster, the better in low light, and obviously less depth of field. it takes alot more to make a constant 2.8 zoom, and it's reflected in the price.

 

it's 1/2 a step. F4 is more than enough imo for prosumers. it's not like you can use the 70-200L f/2.8 indoor anyways. even if you were to use it for a indoor basket ball f/4 is fast enough. with the advance in noise levels for higher iso settings, you're better off bumping the iso imo.

my 80-400 is more than enough for outdoor shooting and I can easily use it for basketball games. in those situations what you really want is reach not faster lens.

i almost bought the d200... my d70 was giving a chr error but I cleared it in 5seconds and it never came back :hs: mostly happy

 

Honestly as long as you don't mind dropping the couple hundred bucks difference in upgrading to a new body and it makes you happy then go ahead. But if it were between upgrading the body and getting a new lens, but not both, I'd go for another lens. What lenses do you have right now?

(Btw, fastest lens doesn't necessarily mean best image quality. Actually the faster the max aperture, the more trick the optics have to be to give good IQ.)