Lessons I've learned playing with lighting setups (and still trying to learn):
You can do a lot worse than beginning with a single umbrella and stand. Soft, pleasing light.
You will need some way to fire your 580EXII off-camera. You can go with a full-fledged radio control system, an ST-E2 or a Chinese knock-off of the ST-E2 from eBay for about half the cost. I went for the knock-off, but I have a 7D so I can use it's on-camera infrared controller as a back-up. (Why did I buy the knock-off, when the 7D has a built-in controller? Because the 7D relies on the on-camera flash which overheats after a 10 shots and then you are screwed until it cools off.) Since I don't do this for a living and don't have anything but personal pride at risk if there are problems, I haven't had the need to invest in a radio-controlled system. I've never had a problem with the infrared working, even in bright daylight.
Buy the Manfrotto/Bogen swivel umbrella adaptors. They cost twice as much as the $15-$17 plastic ones, but they are all metal and will actually hold your umbrella or softbox in place. http://www.adorama.com/BG2905.html
Read the Strobist column on how to use these, so you don't put them on upside down (easier than you think).
Buy some of these http://www.adorama.com/BG2932.html
They are plastic. Maybe I am paranoid but I don't like to put my strobes on metal shoes. Afraid of a short.
Shop eBay for additional lighting goodies like softboxes. Many people buy these for a specific project and then don't need them anymore and sell them. I got a couple of very nice Photoflex softboxes that way.
Read the Strobist columns and also sign up for the Photoflex enewsletter, which is actually a monthly lesson in lighting.
The choice between added 580EXIIs and 430 EXIIs comes down to whether or not you need the extra boost of the battery packs (which are only available for the 580EXIIs). I ended up buying battery packs because they speed up the recycle time and reduce the need to switch out batteries in the middle of shooting.
Don't bother trying to buy strobes off eBay, people pay ridiculous prices for used strobes, often more than what a refurbished model will go for. I don't get it. Better to buy refurbished either directly from Canon or watch for refurbs from B&H or Adorama.
The "Strobies" kits are kind of fun and when you have some extra cash, they are worth buying to got a bunch of stuff to experiment with.
I like plain backgrounds so I eventually bought one of these: http://www.bhphotovideo.com/c/product/303035-REG/Botero_C03757_037_Collapsible_Reversible_Background.html
If you get one, practice folding it up at home, so you don't look like an idiot out in public. (Well, okay, I still look like an idiot, but it's really embarrassing when you can't get your backdrop folded back up and have to drag it back to your car and stuff the whole thing in your backseat)
Order a roll of gaffer's tape from Adorama or B&H when you are ordering something else. You will need it.
Consider getting one of these. http://www.adorama.com/BG175F.html
They are way cool and can be a lifesaver.
Go to Lowes or Home Depot and pick up a bag of plastic clamps or you can get the same thing from B&H or Adorama for about three times the price.
I don't have any experience with the lighting equipment others have mentioned. Frankly, by the time you invest in some 580s and battery packs, you've spent as much as you would on some of these other options. But, most of the items I've mentioned here are things you will want or need regardless of your lighting setup.
As with most things photographic, you can drop hundreds or even thousands of dollars very easily. Or you can go the quick route, which is just to buy yourself a knife and open up a vein.