is SLI worth it?

ok now here is the problem

New cards and new games usually play well together, but when the action gets tough the framerate can drop into the low 40's or even lower.

So.. is the solution to buy 2 graphics cards and run them in SLI?

I was also thinking about Watercooling, but since it would add a fortune to the cost of 2 cards aswell, id have to buy 2 waterblocks which would make it even more expensive.

So its either watercooling and one card, or 2 cards and aircooling.. most people have said go for water, but im not so sure.