From what I've seen the 7D actualy has SLIGHTLY worse high ISO noise then the 50D, but in the higher resolution images make the noise artifacts less apparent at normal print sizes (8X10). However if you start cropping your images they'll be easier to see since you're in effect making them larger. The other downside of the resolution is that if you're planning on printing near the sizes that the images can be printed at you'd need the higher end glass for it.
I'm not sure what the size of the file is from an 18mp camera, but I'm guessing that you'd have to start getting udma cards in order to take advatage of the 8fps of the 7D.
All that said, if you can afford it, get it.
It has a better autofocus system and better metering system. Those two things alone would be a seller for me.