It's probably a good idea to target something under 80% of maximum capacity for a new PSU, since that's where they are most efficient.
Three 8-pin PCIe connectors at 150 watts each plus up to 75 watts from the motherboard through the PCIe x16 slot seems like a
tremendous amount of power for a graphics card. Even if it's not using the maximum that that configuration can provide, consider that two 8-pin connectors plus the PCIe slot would have been good for up to 375 watts, and NVidia's engineers determined that it needed another 8-pin power connector.