Generally buying a second card for SLI is the cheapest option to get more performance, but it's not always the best.
Plus, it's entirely possible your system doesn't support SLI. Please list your PC specs (CPU, motherboard model, power supply, RAM). Your budget is important as well.
A 560 Ti is a really big step up. A regular 560 would be a nice improvement. Or go with AMD and get a 6870 which is a very good value.
Intel Core i5 2500k GHz Processor
Corsair Vengeance 8gb DDR3 memory
EVGA GeForce GTX 550 Ti video card
500gb hard drive
dvd cd writer lg
inwin dragon slayer case
corsair builder series cx 600 psu