I would not try to sli with that psu. According to newegg's wattage calculator, the system would draw 635 watts. It is generally a necessity to have a good bit of overhead with the psu so if you want to overclock or just don't want to worry about having problems, you would be covered. I would suggest trying to sell the 460 and putting it toward a new video card. The other option would be to get the other 460 for sli and a new psu, which would likely be quite a bit pricier. I would recommend the radeon hd 7870 or the gtx 660.