I had a 430 EX for my 40D and it was good, but more power is always better 🙂
The 580 will swivel 360 Deg, while the 430 is limited to only 90 deg to right and 180 deg to left of forwards.
also the 430 EX does not work with the camera menu systems, only the buttons on the flash gun change settings, whereas the 430 EXII and 580 EXII can be controlled by the cameras onboard menu systems (40D and upwards I think, not sure about lower/oldercamera bodies)
A chap called ‘Kerso’ on ebay and most UK photography forums can do a very good price on the canon guns,got me a 430EX for £140 about 7 months ago and the 580 EXII was around the 200 quid mark. may have gone uprecently due to the currency rates tho.