I assume you mean the 430ex2...
My personal take is that the 580ex2 is a workhorse for on-camera flash, but it's incredibly over-kill for off-camera flash, unless you shoot in unique settings where you need the extra power.
I think the first question you should ask yourself is how important remote TTL is to your shooting. If you're okay with controlling the flashes manually, then you can get several flashes of equal power for the cost of one 430ex2. My Yongnuo YN560's have only slightly less power (in terms of available light output) than the 580ex2, but I can get about 6 of them for the cost of one 580ex2. You'd want a set of decent radio remotes as well, but you could throw in a set of PocketWizard Plus3's and still pay less, assuming you get one or two remote flashes.
If E-TTL is a must-have, then it comes down to how much light you'll need from your slave flashes. That's a very complex question to answer. When I used remote E-TTL, the available power of my 430ex2 was not a problem, and now my manual slaves are set at 1/16th at most, so the full power my flashes can spit out isn't really something I worry about. It sounds like your flash use is similar, so I doubt you would need another 580ex2.
The only other thing I can comment on is that if you're planning on getting into event photography, the built-in optical remote system on the 430ex2/580ex2 is not nearly as reliable as a radio system. Not only distance, but also line of sight comes into play, and at many events, you want to be able to shoot in any direction and have your flashes fire. So in the future you could go with a set of 600ex-RT's, or maybe some 440ex-RT's if Canon ends up making them. Or, again, if you can work with manual slaves, there are many other less expensive options out there.