Your PSU is more than capable of a GTX285. I would start looking at port problems. Are you using a DVI->HDMI converter for your HDTV monitor? Are you using a VGA cable?
Can you put the GTX260 (or another card) in place of the 285 to test of the board's slot is working?
Also, breaking the little plastic retention clip probably won't hurt anything, so long as you didn't rip off any pins from inside of the slot along with the clip. You may have a problem with the card not contacting all pins however, as it may not be fully inside of the slot, due to the clip being off. Try setting the PC horizontal and see if that helps at all.