I'm personally against recommending CFX/SLI unless the person is planning to play in 4K, where they're absolutely needed.
It's all beautiful when it works, but sometimes it just doesn't...
Sometimes it does but doesn't scale much, and there are gloriously ridiculous cases when X+1 cards are worse than X for some mystical reason.
Things might change with DX12 and it's multi GPU support, but for now it's all handled by the driver in a very guimmicky way.