If you want to add anoter card then ANY 6600 GT card will work. This has only been possible with the recent release of display drivers from nVidia - previoulsy you had to have 2 exact cards from the same manufacturer, i.e. you would have had to have 2x 6600 GT's from Inno3D! As long as you use the latest drivers which is more important than ever with SLi configurations (read the release notes on some of the versions!). In terms of power it depends on the board and the card. I recently built a system with a 6600GT that did not require an additional power connector and only received it's power from the PCI-E bus. I'm guessing that if you go SLi you will need aditional power to the motherboard - there is usually a connector for a standard molex power connector. What board have you got?