Your psu has adequate power. Since it is working, and running ok on the second pc, you should be OK.
But, be advised that the psu is a tier 5 (not recommended, replace asap) unit on this list:http://www.eggxpert.com/forums/thread/323050.aspx
Install the card in your pc, and attach the power leads. Attach your monitor to the card.
In the bios, you should have an option as to which graphics adapter you want to use on startup. Select the discrete card.
Some bios'es will automatically prefer a discrete card when present, some will not.