....and is this a big improvement (I only say that cos it looks like a newer model)?
The only thing I would say is that the standard lens on the 400D (if it is athe same as mine) is 18-55mm and the second lens a 90-300 so you will have a gap of 35mm. Therefore, you may need to compremise on some shots which may fall into this gap.
The 90-300 lens would be the best buy as it has a USM drive which is quicker and quieter to use. The other thing I'm not sure if you know is as these are not dedicated digital lenses the lens focal length will be 144-480 when used on the 400D as it will be magnified by 1.6 .
I have a 400D myself and find no trouble at all when I use mine.
Peace , Lubin