Here's one of the photos I took at a recent concert in Hong Kong, sitting about 25 rows back from the performer.
Canon 350D; 800ASA, f5.6, 1/100th
manual exposure; settled upon after a few trials and glances at the histogram
Canon 70-300/4.0 IS DO; manual focus, handheld (I don't like tripods)Zhou Hua Jian in Concert 2006
If the 350D can do it, the 400D can definitely handle it. Auto-focus on the 400D is also far better than on the 350D so it might very well be usable for live shows. personally, I prefer manual focus, especially when the DOF is just a few inches or less.
but more to your request of a camera that you could fit into your bag: I'd suggest that you try the Canon IXUS 800 IS or the 850 IS. we have the 800 IS and it's an excellent camera. in manual exposure mode, it can probably serve your needs nicely. it features 6 megapixels, 35-140mm optical zoom and a very nice 2.5" LCD monitor.
the 850 IS features 7 megapixels, 28-105 optical zoom, the same 2.5" LCD monitor and a new DIGIC III processor.
both cameras have an Auto-ASA mode with the 800 IS providing film speeds from 80 to 800 and the 850 IS providing speeds from 80 to 1600.
I'm assuming that you know that the 'IS' stands for Image Stabilisation.
the 850 IS would probably be your best bet unless you really needed the extra 35mm of zoom.