I have seen some mATX sized PSUs that are supposedly around the 300W mark. 'A Power' and 'Arrow' appear to manufacturer higher powered mATX PSUs. Enermax appear to do a 230W and a 270W
PSU too.
A number of the Shuttle systems ship with a 200W
PSU. Just make sure that the
PSU is well matched to the load on the various voltages!