If the XTi you are referring to is new, that is the route I would take. Generally a good, used 30D is running around $700-750 without a lens right now, so the price should be around $800 for that package. A "too good to be true" price on a DSLR on Ebay smells like trouble. Image-wise they are nearly equal. The 30D is better built, but doesn't actually take photos better than the XTi. Buying new, would get you a warranty, buying used there would be none. If you're looking to buy used equipment, you could also try the "For Sale" forum at fredmiranda.com. I have bought and sold $3-4000 in photo equipment in the last couple of years there without ever a problem.
I agree with the 85mm/1.8 what you shoot. It is fantastic for it's price. I had one that I used with an XT and 20D. When I sold my Canon gear, the 85 was the hardest to part with. For sports and dim light you have to have a fast lenses. IS will help if you are shooting something that's not moving. Moving subjects will still blur if you don't have enough shutter speed which you can do with the larger apertures.