I love my 7D but I can definitely agree that 3200ISO and above produces a little too much graininess. On the other hand, some have suggested de-noising software, here's another one which I find to be *the* de-noising app. Noise Ninja from picturecode.com.
You can even download 7D optimized de-noising profiles for it. It works wonders.
As for the longevity, not to worry, you buy what you need, when you need. Otherwise you'll be holding out forever as there's always something new coming, sooner or later. And the 7D is _very_ capable.