Taekwando is such a fast-moving sport and in order to avoid motion-blur, you will need to target shutter speeds of 1/250sec or maybe even 1/500sec.
To achieve this indoors, you need a lot of light or quite high-ISO settings.
You can get more light if you use a large-aperture lens and by adding your own light via flash.
Keep in mind that not all sporting events allow flash. Also, study the guide-numbers carefully on your flash. The built-in flash doesn't have much reach, but your distance to the combatants really determines what you can do with the flash. Furthermore, check the max. flash-sync. speed on your camera. I don't know exactly, but I'd assume that the 40D can't sync faster than 1/250sec - if even that.
You may have to buy an inexpensive 50mm F1.8 lens to get enough light - that is cheaper than buying a flash and those lenses are sharp. However, you have to get close.
From my own experience with some basketball shots - in a school gym - I could get 1/80sec with F5.6 and ISO800 @ 200mm. Assuming roughly the same level of light, you need 4 times the light, 4 times the ISO or 4 F-stops more to achieve 1/320 sec.
If you are willing to shoot ISO1600, you can halve the numbers.
All in all: unless you have a large-aperture lens, you might just be at the edge of what your equipment can do and get sharp images without motion-blur. I assume that you'd like to get some of those flying reverse spin-kicks captured here..lol.
Then again, the venue may be a lot brighter than the example I gave you. If it's a professional venue that usually accommodates professional photographers, there is a chance they may allow flash and that they have more light.
On the other hand, using flash severely limits the frames-per-second you can shoot, due to re-charge time and you may need to shoot series on full-auto to get that perfect extension and knock-out kick captured.
Regardless, you may consider using manual focus so you don't have to wait for sluggish auto-focus to react to your input. I'm assuming that you of course will use a tripod?
Good luck with the shooting! I hope to see some pictures from the event?