As the numbers indicate the card is 8.3% slower. Not a huge deal but you should be able to easily put the card back up to its normal speeds using software such as rivatuner; http://www.guru3d.com/index.php?page=rivatuner
You cannot connect your card to your PSU. It doesn't have a connector to do so. If it does then it should definitely be plugged in.
The difference between the two should be nearly nothing. 50MHz slower on the core and 175Mhz on the shader doesn't amount to much. I wouldn't worry about it.