the only advantage you get of a crossfire setup is you don't waste the current graphics card. the whole money you spend goes into an upgrade instead of losing the current card.
with that said, any single card config is better on the long run.
so my advice... if you can play all the games you want now at max resolution and good enough quality (and on 1080p you would) then just wait it out. you'll either be able to get another 270x later on at a better price or upgrade to another card entirely again, you'd buy a better card for the same money.