Ok, a new gtx650ti boost will run @$200. A good 550-600w psu to run sli will run $60-70, New shrouds to cool (if you can find them) $150. That's a lot of money invested for so little return.
For the sli you'll need a decent 550-600w psu. You could get a gtx760, which will perform the same if not better than the 650ti in sli, for the same $200, and only requires a good 500-550w psu, which you probably already have. If not, toy can get one for $30-$60. The 760 will run cooler, quieter and will not need an upgraded case cooling since there is only 1 gpu, not sli. Either way, performanc-wise, upgrading the gpu makes more sense to me, price-wise, the same.