450W is 82% from 550W, depending on your PSU that might mean that this may exceed recommended continuous output (most of the PSUs are built for long-term operations at <80%). As well as your PSU will not be as quiet as 650W PSU as at 80% PSU fan noise is highest. This also means that you almost won't have any headroom for further upgrades.
Also the difference between 550W and 650W is about 10EUR, which is insignificant.