SLI (NVidia) and Crossfire (ATi) both require the same video card GPU, so in your case you would need another 250GTS for SLI, as well as a compatible motherboard that can support SLi. Just because a board is rated to be Crossfire compatible, doesn't mean it's SLi compatible, so make sure your board is SLi compatible.
Actually in Crossfire (only) you can crossfire two cards in the same series, say for example you can crossfire a HD5850 with a HD5870 if you want. Doesn't have to be the same card, just the same series, However with SLI you need the same cards.
@OP ohsonoob has a point about making sure your motherboard is SLI compatible, Also make sure that your PSU is up to the job.