do you think this is a good idea to build another pc?

Hello I have an older pc that is useless and takes up a lot of energy when in using and I am trying to conserve energy so is changing from an Intel Core 2 Quad Q8300(95W) to Intel celeron 1840(53w) a good idea? I mean I can see it saving more energy when in use but is the saving that much? Mostly use the pc for web browsing/ watching movies on. I got the TDP from intel site. I guess the rest of the specs don’t really matter but the current older computer has is decent for whatever is needed to do on there.
Specs:
Intel Core 2 Quad Q8300
8 Gigs DDR2
500GB HDD
400 watt ps
Specs
Intel Celeron 1840 4Gigs DDR3
I am going to use the same hard drive and power supply etc. My question is do you think its worth it? Only about $223.84 including OS windows 7. Also is this a legit seller that sells real oem windows 7 on ebay. He has 370 postive and 1 negative feedback. http://www.ebay.com/itm/Microsoft-Windows-7-Professional-64-Bit-SP1-OEM-Full-Version-Upgrade-PC-/251845025680?hash=item3aa3221f90
Answer #1
Not worth it. You are not going to save the 150$ anytime soon and no reason to upgrade for future proofing the system cause both cpu have about the same performance. I would never do it cause it’s a waste of money.
Answer #2
Wattage doesn’t mean jack if the CPU isn’t efficient.
A powerful processor doesn’t have to run at its full speed and hence full power if the workload is not significant. Where’s a low tier CPU will have to run for a very long duration on its highest clock speed, when given the same workload.
Answer #3
As said, no significant savings..
Keep the old girl and turn it off when you’re not using it!
Answer #4
Electricity is sold by kWh. So look into it, will stop you from making silly decisions. And since PC’s have non-linear power draw characteristic, will be fun calculating or even measuring it.