Check the actual wattage on the 12v rails and ignore the stated capacity of the psu. A lot of psu's make up the max power on the minor rails were it isn't really needed. Your VP450 for example is supposed to be a 450w psu but can only deliver 360w on the 12v rails where it counts.
This is a excellent place to check specifics of psu's such as max power on 12v rails, who actually builds the psu and links to reviews.
Personally i wouldn't buy anything that OCZ makes. They are in serious difficulty and may not be around much longer. If they go under that's the warranty gone.