See comparison below:
They're actually really close. I don't know which 5870 you have (1 or 2gb) or which 580 you would be looking at (1.5-3gb) but it would only be a small step down. You would also save on power consumption, not to mention probably heat - depending on your setup obviously. If you have the two cards sandwiched (ref vs non. ref coolers will make a difference here). If you're using waterblocks for your 5870s and plan on cooling your 580 with water, might be an added cost.
Speaking as someone with 2 5870s also, I think they are beastly cards and I have no problem running anything out there maxed out right now. I had a few issues with Skyrim, but nothing else. I would recommend doing a cleanout/reinstall before you make the leap to a new card, as you might be able to remedy the issues for 'free' without too much headache.
In the end it's your decision - obviously being able to upgrade to SLI 580's down the road would be worth it!.. Keep in mind if you have more than 2 monitors (if you're doing eyefinity right now) that it won't work with a single nVidia card - you will need at least 2 cards to support nVidia surround, barring certain special card setups.