I'm in the same situation. I had a 350d and a 40D previously (currently have no camera). I can really appreciate the better usability of the 40D and by extension the 7D with the dual dials, and top mounted LCD, and the ability to change things quickly.
However, I can get two 550Ds for the price of a 7D. Thats either a lot of savings, or a lot of money for glass.
And if you don't need to rapidly change settings (realistically thats me at least), then the 550D seems a perfect compromise.
The only thing that gives me pause is the knowledge that the 7D will handle better, and will have better/faster AF. And so my geek side knows that it wants the best option. But my practical side (not usually strong) says get the 550D ,get some decent glass