Are you certain your motherboard supports SLI?
If so, I'd add another GTS 250, keep the X3 720, and my next upgrade would be the entire computer.
My only other recommendation would be to hold off and get a next-gen NVidia or ATI 40nm when they come out that performs better than the HD4890.
Either method you recommend involves throwing away a fairly good part which seems a waste to me. Your methods provide very small gains for the money. Anyway, you have my recommendation.
Games that better support SLI will of course work better. I think my solution is the best bang for the buck with nearly every game that needs it.