either the lens first or body first is up to the OP's piority and preference. One of the things that i'm going to throw into the pot is the fact that 50D has been out for 1.5years already, soon some new camera may take over and the value of the 50D will drop by a portion. But put that on a side,If you are going to get both anyways, then I would ask the OP which of these two can give you "the shot" that you need earlier. If you have a slow lens. A better IQ, more DOF control and faster shutter speed can get you "the picture" better and easier, then then by all means, get the 70-200mm. But if the higher ISO quality, faster frame per second, microadjustment, better built, two wheels, etc. along with your current lens can get you "the picture" better and easier, or if getting a "bigger" body will get you the confidence/revenues, then by all means, get the body first.
But as a general rule is always lens first and body second, but if you've already has the lens that can do the job, then go for the body. One of my argument for me if i have enough lens is that: upgrading one lens can only release the potential for a body(if you have only one body), but upgrading a body can release the potential for all your lens(if you have many lens).
Anyways, have fun!