If it's a sandy bridge processor, you will get much more mileage upping the cpu multiplier, as speeding up the core clock tends to cause it to fall out of sync with other systems, and become unstable.
Even on a super-powered I7 with a ton of ram, 580GTX, you will only see perhaps 500W on maximum load. Over clocking the processor has a minimal effect on overall power consumption. You might see a 10-15% increase in energy consumption for a super high over clock (Vcore ~1.4) Your processor is rated for around 130W, +15% = ~150W. Total draw of 520W means you will see a 3% increase in overall power consumption, for a significant increase in processing speed.
Only downside of overclocking is it tends to make the computer a bit more unstable, this myself is something I cannot have since a lot of the things I do require my computer not crashing while I'm in the middle of it. But for most people an overclock can help you quite a bit if you are feeling restricted by your computer.
On the other-hand the I5-2500 overclocks so well, and can give you such a significant boost in power (equaling overclocked I7's in many cases) and the I5 costs a lot less. So it would almost be a crime to not overclock a processor like this, as it can save you hundreds from not having to buy a more expensive processor and still get similar performance.
Leaving your computer on overnight just to idle and do nothing is pretty wasteful, but I doubt any of you use your computer to its fullest extent even 5% of the time you use it. Unless you happen to do a lot of video rendering or intensive calculations. So your estimate for $200 a year is awfully high, I'd expect it to be more around $40-60 a year.
If you want to get a serious savings in power, just switch your lights to compact florescent, or even better LED. 6-15W vs. 100W. Talk about inefficient, a Incandescent light only converts 10% to actual light, the rest gets converted to heat.
