I'm looking at flashes right now, found a couple of cheap Sigma's on ebay that will do the job.
Mainly it's the EF530 DG ST I'm looking at which is the cheaper version, it's only really lacking the ability to trigger an extra flash compared to the EF530 DG Super, but I can live with that. Both have a Guide Number of 53 but you can find the DG ST for less than £100 on the bay. The Sigmas do iTTL and auto-zoom even on the cheaper DG ST.
The newest model from Sigma is the EF610 DG ST for about £140
(here) or DG Super
around £230. But that's still cheaper than a Canon 580EX II.