An increase in "capacity" just means the new PSU will have more in reserve so thats a healthy position for your PC to be in and no problems there! and there would be absolutely no detrement going upto a 650watt PSU if you wanted but that would just be overkill for the average PC demands.
Quality! as others have mentioned, is much better than quantity, so spend a few extra $$$'s for peace of mind.
Martin
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