The 5870 XXX edition is an overclocked edition.
If you're paying more than $10 for it versus other 5870's, don't buy it. It's easy to overclock them yourself.
As for performance, usually 4-8 FPS boost in overclocked version.
Don't forget the fact you'll get better support if the overclocked card has issues. Buy a stock card and overclock it, you may not get any warrenty service at all.
won‘t bet on the extra support, in fact special picks of bins would means longer wait time if ever to RMA, as the manufacturer would now need to return card with silicon of a good bin (spell: rare).