Depends on the intended use. For birding and wildlife, which are what I'm guessing the OP is intending for this lens, 400mm is sort of at the short end of what would be used in the wild (ie. not the zoo or parks where you can get close to the bird/animal).
Reason I asked about a budget is because the 2 main lenses I would suggest, and that fit the OP's guidelines, are in pretty different price ranges.
Canon 100-400L IS ($1200-1300'ish used)
Sigma 150-500 OS ($800-850'ish used)
Both very good lenses with stabilization. Since the OP has a 7D, there's no zoom lens that goes to 400mm or longer that will AF well with an extender attached (hence the 1D3 in my signature which does AF in that situation ). You can "tape the pins" on a Kenko 1.4x and get it to AF with these lenses, but the AF will be noticeably slower and less accurate than without it attached (assuming the 7D performs the same as my previous 40D and 50D did with that setup). You can manually focus with the extender attached and get pretty good results, but that's tough to do if the bird/animal is moving much, at least for me it is!
Hope that helped a little.