I did the same thing with a very similar P4 computer. It does eat some electricity, but it is substantially faster and has more inputs and outputs than an original Model B Raspberry Pi. This could make for a more enjoyable experience learning to use a Pi since it can use USB persistence and be utilized on nearly any PC.

Since most of the software is the same it could be a great way to make up for a a lack of funds to fill a classroom with Raspberry Pi machines. Nearly any donated PC that still runs could be made to work even cheaper than buying any of the Pi computers. People could rotate so if they want to experiment with the GPIO pins or other Pi specific components they can have a chance, while other won't have to sit around and wait for a Pi to become available.

The new version of Raspbian x86 Stretch should be coming out very soon.