I shoot various "event" type photography for outdoor camping, scouts, school events and swim meets. Getting a fast lens will only go so far with low light and it won't work miracles. Even if you can get focus lock, you still have dark faces, etc. It always helps to have a small bit of fill flash. Also, consider getting a 5D body (if you're not flush with cash) or a 5D2 so you can utilize the fabulous full frame sensor it provides. It's better with low light than a crop sensor. Then, get a 16-35 f/2.8 L lens. You don't need the version II. Couple that with a decent diffused flash and use flash compensation to dial it down so people don't look like crime scene victims all washed out with a dark background because you're up close. Try using Av on Auto so backgrounds look natural but this will lower the shutter speed so the flash will help with focus unless your subjects are moving a lot. You'll also need to bump the ISO. If this isn't your style, fine, the background will just be dark.
If you decide to get a fast prime, the 24 f/1.4 L is a good lens for both the FF 5D/5D2 and the Crop 60D. (24mm or 38mm) Nothing wrong with the sweet 35L but it gets a little too long on the crop for my taste. I work wide and get up close with a shorter flash duration. I actually use a SunPak RD2000 with a StoFen diffuser most of the time with it dialed down 1 - 2 stops or more. It's small and light. The darker the area, the less flash you need. The Gary Fong is a great diffuser for weddings and stuff but I hate walking around with something like that, esp if you are carrying two bodies. Just get a Sto-Fen diffuser and you'll be fine. Or if you plan to carry two bodies, put the SunPak RD2000 on the candid body and the big flash with the Gary Fong on the other one you plan to do bigger groups with. Either the 580 EX or the 580 EX II will work great for you. Heck, even the 430-II and the older 550 EX II are still excellent flash units. Just don't forget a good diffuser.
Anytime you are shooting available indoor/low light, plan on shooting like crazy with multiple exposures to increase the chance of getting some better shots. If you're fairly new to available light, esp where people are moving, I strongly suggest you start early and practice, practice! You can also lower your exp compensation sometimes to quickly give the shutter speed a small assist when you're in a bind but make sure to put it back!
Last, get Lightroom and shoot RAW. You'll be amazed at how many shots you can bring back from the dead if you are willing to live with a little noise. The only thing you can't fix is poor focus so that's where the practice comes in figuring out what setting compromises you can make for better focus at the cost of bright exp, aperture or ISO.
Good luck and have fun!!!