For level occupancy, the aircraft has to fly at +/- 200 feet accuracy.
However, for system performance the RVSM MASPS require that the aircraft type Altimetry System Error, for the basic flight envelope (JAA TGL6 §7.3) should meet the following:
(a) At the point in the envelope where the mean ASE reaches its largest absolute value that value should not exceed 25 m (80 ft);
(b) At the point in the envelope where absolute mean ASE plus three standard deviations of ASE reaches its largest absolute value, the absolute value should not exceed 60 m (200 ft).
Full information here:
http://www.eur-rvsm.com/