Alright so here's the current pricing in my area:
$120 for a 5770
$260 for a 5870
Should I jump into this mining boat headfirst and buy a 5870? Or is the difficulty increasing so fast it won't be worth it to recoup my investment?
http://bitcoincharts.com/charts/ seems to indicate it'd go down next change.
Your purchase should depend on the price of the rest of your rig, PSU wattage, likelihood of buying a new rig/new parts and acceptable noise/heat generation levels taken into account.
Recouping your investment isn't assured, but you will offset the devaluation of your hardware over time.