Are you ever going to shoot on location to where you need battery power or portability is a huge option?
You can build kits on a budget. You can find new manual flashes for about $100 that would do almost everything that 580EX II would do off camera with a radio trigger stuck to it. If you don't want to have to keep track of batteries, you can look at something like Adorama flashpoints. You can get a 150w/s monolight for $100ish. For the price of 3 of those and triggers, you'll probably be at less that the cost of a 580EX II.
Don't get me wrong, I own TTL enabled hotshoe flashes and I think it's good for most photographers to have at least one in their kit, but if you're wanting to do off camera lighting and don't need the TTL feature, then it's not worth spending the money on the flashes when you can get lights that do everything you need for a fraction of the price.
Oops. OCF - Off Camera Flash. A set of triggers will benefit you more. There's some times that all I have with me is a hot shoe flash and I don't want a direct harsh light so I toss the triggers on it and set it up in a corner of the room to bounce of the celing and provide a nice soft light.