The main difference is the 30% faster recycle of the 580EX-I compared to the 550EX... along with a completely, more intuitively built rear menu system. Also, the 580EX-I has a few Custom Menu Settings unlike the 550EX along with being able to control the 580EX-I flash through the camera's menu setting.....
Typically, a used 550EX will go for about $150-$175 used and a 580EX-I will go for about $250-$275 used and a 580EX-II will go for about $325-$350. Basically about $100 difference between each model.....
Now here is where I throw in my *shameful* plug..... I have two Canon 550EX Speedlites for sale. Both come with the Original Box, Leather Pouch, Table Stand, Manual, and Warrantee Card. Selling for $175 ea. (includes PayPal Fees and Shipping). I also have a Canon 580EX-I for sale. Comes with just the Leather Pouch and Table Stand. Selling for $250 (includes PayPal Fees and Shipping). All flashes are in excellent working condition and have the normal scuffs and scratches that one would probably see from a flash of this age.