I'd rather see you spend the money on a Canon lens with a fast aperture, then pump the ISO up a bit, and natural light will look much better then anything you'll come up with with flash. I agree with what Jim said exactly.
All of my stage photography (theatre) has been with fast aperture lenses (even 2.8 will be fine if you can push your ISO a little):
http://www.flickr.com/photos/brianhirschfeldphotography/collections/72157625730354453/You may also find my article "Please buy a 50mm" of some interest
http://brianhirschfeldphotography.com/2011/12/07/please-buy-a-50mm-lens-2/Depending on how far / close you will be here are some great options which probably cost around whatever you'd wind up paying for a flash....if not less....
Canon EF 50mm f/1.8
http://www.bhphotovideo.com/c/product/12142-GREY/Canon_2514A002_Normal_EF_50mm_f_1_8.html Whenever some friend of mine has a birthday, and they are a Canon shooter, even if I am getting them something else, I will stick one of these in there with it.all around great lens, and it will have some extra telephoto pull because of the crop factor on your sensor, it will be equivalent to an 85mm.
Canon EF 35mm f/2.0
http://www.bhphotovideo.com/c/product/12119-USA/Canon_2507A002_Wide_Angle_EF_35mm.htmlthere is really no difference of consequence between 1.8 and 2.0, and if you are going to be closer this lens will act as more of a real 50mm on your crop sensor body.
Canon EF 50mm f/1.4 USM
http://www.bhphotovideo.com/c/product/12140-GREY/Canon_2515A003_50mm_f_1_4_USM_Autofocus.htmlthis lens is better then the 1.8 but you also pay more, of course 1.4 is better then 1.8, and it will be again equivalent to an 85mm and has a USM which like Nikon's SWM leads to quieter autofocus.
Canon EF 85mm 1.8 USM
http://www.bhphotovideo.com/c/product/12182-GREY/Canon_2519A003_85mm_f_1_8_USM_Autofocus.htmlHas the USM, and will be a 135mm 1.8 telephoto lens on your crop sensor body, which will be great if you are further away or want to get alot in the scene.
Canon EF 100mm f/2.0 USM
http://www.bhphotovideo.com/c/product/12058-USA/Canon_2518A003_Telephoto_EF_100mm_f_2_0.htmlI have never used this lens since I shoot Nikon, but I have hear excellent things about it, its fast, and a medium telephoto on your sensor weighing in at 150mm so if you are further away or want to get closer in there ya go.
now if you are going to be really really close, or you are going to want to get more in the scene (a group of people, a whole stage etc), then you might consider something wider then 35mm (50mm on your sensor really) but a 50mm you can't go wrong with but if you know the distance then you can figure out how much pull you will need and know if a 50mm is going to be enough. You can most likely simulate alot of these focal lengths with whatever lenses you have now to get an idea of what kind of reach they will have, but not their performance optically obviously.
As you can see all of these options are below 500 dollars, and some of them are even cheaper then Canon's least expensive TTL flash, the 320EX, and
ALL OF THEM will certainly give you better results then using a flash.