I think the kit lens is 18-55, which would be okay if you can get within 15-30 feet of your subject.
I take track and cross-country pictures, and I can get pretty close to the athletes, so I sometimes use my 17-35mm tamron zoom.
You could try using your kit lens, but if you're not getting enough reach, consider getting a longer lens for basketball.
Some people use the Canon 85mm F1.8 prime lens for basketball, which does anice job indoors.
Another nice all round sports lens would be the Sigma 70-200 F2.8 lens which you could use indoors as well as a lot of other uses.
You could add a teleextender to get even more reach from the 70-200.
As for flash, I have the Sunpak 383 which is a reasonable flash, especially if used as a "fill flash".
I bought a Stophen Diffuser for it, and then I just use it manual at 1/16th or 1/8th power to throw out some fill flash.
Sometimes I use the built in flash if I'm taking pictures of athletes within 20 feet, and the 20D built in flash does a nice job of fill in flash.
Again, not sure of your price range.
A buddy of mine uses a Canon 70-200 F2.8 IS lens on a monopod. It's a big, heavy, expensive lens, but it's truly a dream lens.
Another cool lens is the Canon 17-85 IS lens, which B&H photo is selling with the 20D for about $1250. That's a great deal.
Better to get the 17-85 IS than the kit lens, in my opinion. You could probably get by with it for basketball.
Although the 17-85 lens is F4, I've used an F4 lens indoors, and if it's a fairly well lit basketball court, you'll get good pictures at ISO3200.
-- Terry