well so far the 420EX is not a bad match for it.
You can try the Sigma EF500Super DG version tho I'm not sure if it work on it. The version b4 it, the EF500Super doesn't seem to work on the G-series and Pro90IS. Maybe the DG(digital) version do support it. You can oso consider the Metz flashes(Sorry I'm not very info abt Metz).