hey Alex,..one of the moderators gave a link to a site that computes PSU capacity/wattage calcutator..
IMO 400 watt PSU is enough,..but personally i want to have that extra headroom for future expansion. but with your current hardware and periphrals,..400 is good.
i'm a little concerned about the power supply that you're getting,..for a 400 watter,...its should costs no less than 40USD, IMO. the PSU quality is important, so i suggest that you get a more reliable one...probably an enermax psu.
i also read somewhere,..that to be sure youre getting 400 watt or more,..the psu should be heavy.