Simply, your not going to see a difference on your electric bill.
Normal use, you prolly never see your E2180 hit and maintain 65W TDP. One program that can touch the TDP, is prime95 when it's doing the FPU 800000 iterations. I normally see hottest temps when its doing that test. So even when prime is running, its not going to push it to its TDP constantly.
Whether your machine is idle at 1.2ghz or 2ghz your wattage usage will be low enough not to really make that much of a difference. The main purpose I use speed step is to get cooler CPU temps, and perhaps lowest vcore.
And besides all that, if you pair a 8800GTS (or similar) with that 2180, you might as well kiss good bye to any power savings, even at idle.
business desktop idles around 100 watts for 17" lcd monitor and box, then you add video cards, drives, second monitors, etc.
the savings are not significant until you add up the whole picture at idle. box, monitors, speakers and accessories, turning them all off except my box at idle with speedstep will go down to idle at 85 watts at 120v through a UPS. turning off speedstep idle is at 117 watts. entire system with 2x22" monitors on and speedstep disabled= 296 watts at idle. using the system peaks over 400 watts out the plug without stressing the video card. just tool around with that information and save power if you like.
You can also lower your cpu voltage manually on some boards in the bios. I got mine down to .975 volts on a celeron and windows still ran pretty stable, and speedstep still works. But I think the biggest power user is still the video card. I use onboard video or a low end card; I'm not a gamer so I don't need an 8800gt.
Heh.. if memory serves, my old P4 3ghz paired with 6800GS AGP idled at 115-120w.
My E4400 idled at 109-111W with a 7300GT (my dad's old video card for testing).
When I hooked up the 8800GTS 320mb (G80) it idled at 160W, with the E4400. Then I got my Q6600, and in the same process, replace the G80, with the 8800GTS 512MB (G92) and PSU Corsair 620W. From there, my system idled at 150W.
Of course having a load on it is a different story. I use the P3-kill-o-watt meter for those past readings. So even having a Q6600, my idle temps are not much different from my E4400. And using the S3 power management, putting the system asleep also helps power consumption to a min, as well as bring up the system faster.
Since I use 2 UPS's, my total power draw from everything is around 420-500watts (load).
the range of power numbers i gave was a q6600 at 3.2, x1950 pro, 6 hard drives, dvd burner, tv tuner, total of 13 - 120mm fans. Logitech 5.1 x530, viewsonic and envision 22" monitors, 2 cheap powered usb hubs, keyboard, mouse, whatever. it all adds up.