This might help a little bit...
I always recommend trying to get the utmost out of your equipment before giving up on it and buying additional gear.
You have overexposed your subjects slightly. This often happens when shooting stage lit subjects because the camera will try to evaluate and compensate for the entire stage being dark. If you cut the exposure a stop or so, you could have increased the shutter speed.
Your 40D camera has a spot metering capability, I would use the spot metering which will provide more accurate exposure for the actors. Also shoot with "Auto Exposure Bracketing" in a three shot burst mode while using Aperture priority metering. This will give you several shots at different shutter speeds. Additionally, often the middle shot of a three shot burst will evidence the least camera shake.
Boosting the ISO to 800 or even 1600 might give you a faster shutter speed. The 40D does a pretty good job at high ISO settings and you can reduce any noise in post processing.
If possible, a monopod will reduce camera shake.
However, it might be time for a new lens or two...
The 18-55mm IS kit lens has a variable aperture and it stops down to f/5.6 darn quickly as you zoom in. This is pretty slow for stage productions. If you intend to shoot a lot of stage stuff, it might be worthwhile to invest in a faster prime lens or two which will allow you to maintain a faster shutter speed.
The 50mm f/1.8 or even better the 50mm f/1.4 would be a fairly good lens for medium shots of the stage while the 85mm f/1.8 would be great for closer shots of the actors. Neither of these lenses will break the bank in cost. Of course, when you are shooting at f/1.8 or so, the depth of field is pretty narrow (there's no free lunch in photography).