That is not true, there is a certain amount of money, depending on what you plan to do with the PC, you can spend and get a good performance for your money. When you exceed that limit you will end up spending disproportionally more money on the less performance. And thats the part where it does not make sense to cross that limit.
If you play in 1080p, 1 card is enough. Consider buying 2 if you run 3 monitors e.g.
When your system is outdated, a Crossfire / SLI won't save you from needing to upgrade your system. Consider that the two cards doubled your power draw in that time on the other hand.
Sorry for my crappy english sometimes :/ I hope you get what im trying to say.