The 2D settings are already underclocked and undervolted and underclocking even more might result in unpredictable results. Try and see though, because just a couple of days ago, I got the same idea with my CPU.
I normally idle at 32-34c depending on ambient. I also normally have my fans running at 70% at idle. I grabbed RMClock and enabled Cool&Quiet in my bios, configured my CPU to run at 1180MHZ @1.18vcore idle, 2950MHZ @1.39vcore max with RMClock.
My idle temp is now 27c with fans at 50% which means they can hardly move dust, much less air. Very quiet! My fans automatically ramp up to 100% based on mobo (PWM) temp, then go back to 50% rpm when things cool down.
Sorry for the derail, but RMClock is a really cool program! You may want to check it out since you are interested in saving energy or quieting your rig.
You could mod your vid card bios for a lower 2D clock with nibitor or underclock just your 2D with rivatuner as well.
Abit IN9 32x MAX- Kentsfield QX6700 @3520
1.5 vcore watercooled D-Tek Fuzion/PA-160/MCR120/2x MCP655
2x2GB G-Skill 1066 5-5-5-15 2T@1.9vdimm
2x EVGA 580GTX 1.5GB Sli
2x 74GB Sata Raptor Raid0
2x 320GB Hitachi Sata II
X-FI Elite Pro
Lian Li V2100B