You can't do SLI with Nvidia cards of different models unless they're the same card with a different name (this is not the case here). Also, low end SLI such as GTX 550 Ti SLI (it's 550 Ti, not 550) is more stutter ridden and power inefficient than buying a new single GPU card. I recommend selling that GTX 550 Ti and buying a Radeon 7870.
Hmmmph. That's tough to swallow. I just got the card, and for a solid price. The 550 Ti is definitely a budget card. I will have to think about that for a little while. Thank you for the quick and honest reply.