When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.
Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.
A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.
If you REALLY want to learn a WHOLE LOT about power supplies go here and read this >
http://www.hardwaresecrets.com/article/Anatomy-of-Switching-Power-Supplies/327
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.
Hope it helps.
At least that's my jist of it.