I don't think the parent is suggesting that you buy components to replace fully functioning and useful parts just to save electricity. Potentially, though, you could save real, actual money buy buying newer parts than upgrading your current, old hardware.
I ran an 8800GTX until it died, but it was around 6 months ago and I decided I needed an upgrade (before it failed). If I had gone ahead with the upgrade, I would have paid £100 for the card, and another for a 1kW PSU to handle the draw. Those cards pull north of 320W under load! Thankfully (?) it failed before I upgraded, so I went with an AMD HD6950 instead, and haven't looked back. Performance improvement is wonderful, power draw is down 50%, and I didn't need to upgrade the PSU (meaning the £200 budget could go on the card).
However, having just also upgraded the bare-bones too, I can safely say that the biggest power saving you'll make is upgrading to an SSD. Power draw isn't the issue; It's the fact that you can go from power-down (hibernate or cold start) to working in ~20 seconds. It makes sleep and low-power (but still working) states pointless, so you'll power-off almost every time you leave the thing for any period of time. Again, only if you're looking to upgrade, but worth considering.