No don't go 550 Ti those are a bit underpowered I would go at least Single 560Ti and SLI later if you cant do both right now.
2 x 560Ti in SLI will supply me with what I need?
also I've been reading that SLI doesn't work with FSX or something is this true as I dont wanna waste my money
It is somewhat true FSX gets more power from very fast processors. I would suggest you go to the some FSX forums and see what works best for folks then come back here and we can give you help from there.