The 550Ti is not a very good card to be overclocking and with it already being clocked past the reference clock of 900mhz , the card is currently running at 951 mhz.
What you can do is load the software program Precision-x , which is on the install cd that you have and that program can be used for overclocking and setting the fan to compensate for the heat. The program also allows you to add voltage to the core to help in the overclock.
I would read up on some benchmaking sites and what they were able to get overclocking that card.
These are some good site to visit and get some info from before you start.