Energy-saving tactics for an old PSU?

I think you should install a tool like MSI Afterburner and downgrade a bit your GPU clock speed (50-100Mhz less should be enough).
Don't mess with Voltages unless you know what you are doing.
It actually saved me more than once before ruining old VGAs VRAM.

About the only thing you can do is make sure its dusted out now and then, and disconnect stuff you are not using. Other then that I don't believe there is anyway to reduce the power usages of a PSU unless you downclock the CPU and videocard which will make your gaming worse.

Your best bet would be to upgrade to a Plus 80 Gold or Platinum power supply. Go with a 500 to 800 watt one that way it gives you lots of room to expand in the future if you actually believe its caused by your power supply.

Basically it sounds like your machine is over heating. How often do you clean it out? I mean physically take it out side, open it up and with a can of air blow out the insides of your system? I usually do mine twice a year. If your case sits on the floor and you have pets, it could be filled with pet fur.

Are you sure all your fans are working in all your PC parts that have fans?

If your PSU is that old that it's causing problems then I highly advice you to replace it, not only can it completely fuck up your rig it might burn your house down when you're not around.
I've had the same problem as you in the past and I was too stubborn/lazy to replace it (which in hindsight was ridiculous PSU is one of the cheapest parts after all) then one day my PSU fan decided to bail on me and if I hadn't been around it'd have burned down my house.

Save yourself bigger problems and just get a new one, the risk isn't worth the 10-20 bucks a new one would cost.

disconnect the fans, reroute power to graphics card

Nvidia Control Panel, There should be Power Management Options, And other Place to look would be Windows Power Management Itself.

If all else fails, Then replace PSU. That's about the all i can suggest right now.

PS: Straight to the Point. No, But, This, That.. Give the guy a straight answer.

Thank you all, guys! I'm really looking forward to replace the PSU for something like 750W, but the fact is here in Brazil those parts are really expensive. I'm aware of the problems of pushing it too far, but I'm holding my wallet a little bit for a month or two. And I'm father of FOUR kids, so right now my moneys goes elsewhere lol.