I use that combination and it works pretty well. FWIW, when I bought the 70-200 I did some unscientific comparisons of the 70-200 + 1.7 vs the 80-400 (handheld, in-store). I found the 70-200 was comparable, if not slightly sharper than the 80-400. Of course, focusing was still faster than the 80-400, and if you go with the combination of lens/TC, you get an unbeatable lens when not using the TC
If you don't want to spend that much money, take a look at the 70-300 AFS VR
that just came out. It doesn't have the wide aperature of the 70-200, but the quality is pretty good, especially for the price. Or, keep saving your pennies for the 200-400, it's worth it!