Agree with the other guys: Get a 4650 instead, it's a similar price and much faster. Do not worry about the card being 'too fast', you can increase the AA/AF for improved image quality without sacrificing framerates
Yes, the card will run perfectly on your exising PSU, in fact others have run 9800GT or HD4770 cards on it so the HD4650 will be no problems at all.
You'll have plenty of power to spare to upgrade the RAM and add a second optical drive if need be.
Yes, the converters work, but image quality can suffer, so look for a card that has a built-in VGA output.