Yes, your camera and single-lens combo would fit in a bag like this just as long as you are not using any kind of accessory grip which would make the 20D taller and not likely to fit. It's a holster bag that is designed for someone with minimal gear (i.e. only one lens). It would not be a bag that I'd recommend as a main bag. Instead, go with a larger bag with the capacity to hold multiple lenses, filters, a flash, and other accessories. This kind of bag serves as your main storage solution. The holster type bag comes in as the bag you use when, say, traveling. You load up the bag with the body/lens that you want to use that day, while leaving the bulk of the gear that you do not intend on using back at the hotel.
Holster-type bags are fairly popular among those who only own one lens and want to travel lite. Downside is that people often grow out of such bags the instant they purchase another lens or need to carry an external flash.