You can use a manual flash, but then you would need to do all the calculations in your head for guide numbers and such.
If your serious, get the 580ex. It will talk to your camera and lens and give you reliable exposures, very important. Also it recycles amazingly fast and is just made to be a work horse.
If you cant buy one, rent it. If your only going to do a wedding for a friend, then rent. It will be worth the expense.
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