If you're talking about CPU voltages under load, then two things may contribute to what you are seeing.
First off, if you have cool 'n quiet enabled, Your processor is switching between P-states depending on the load. Each of these P-states will have a different voltage setting, probably...
LGA really isn't any more durable than PGA, infact in a lot of ways it's less durable. For LGA, the pins are in the socket instead of on the CPU, and they're way more fragile. I'm pretty sure an LGA socket is much more expensive than a PGA socket, too.
Even if you mix AMD and Nvidia, it should default back to the windows drivers. You just need to uninstall and use a driver cleaner, and then install the correct drivers.
Dan is correctly about the unified drivers within a brand though; if you're talking about two (current) nvidia cards, they...
6870 or 5850 would be in about the right ballpark.
Do you mean cooler/quieter than the GTX280 (cuz just about any current card at that level is), or subjectively cool/quiet for something in that performance range?