If you are able to get another GTS 250 dirt cheap then knock your self out but the 1156 platform isn't very good for sli/crossfire. A 460 will be a better investment.
PCIe running at 8x for sli is more than enough, especially with a gts 250... But yeah I would rather have a 460 too. Although if you've already got the 250, just grab another imo.