7D is a pointless buy from the 400D and given the glass you've got is cheap it's really going to show how hungry the 7D sensor is.
This will be be apparent on your high ISO shots as canon have crammed a lot onto the APS-C size sensor, if you want good low light performance then you need a Nikon, on the other hand if you want HD video buy a Canon or a video camera

If you're going to get a new body then, like stink says, get the 50D or go full frame.
I'd say the D300s over the 7D any day, but would require you to replace your glass.
So IMO get some quality glass, also I'd recommend buying full frame compatible glass, otherwise when you do want to go full fat you could be stuff with incompatible glass.