I do not know 100% (this is not something folks do very often). But my guess would lean towards no…due to the quad core nature of the "good case" you mentioned…and because of the liquid cooling system in the quad core Powermac G5 model.
The quad core setup and the liquid cooling system may be mounted differently than in the dual-core with standard cooing fan setup. So the dual-core components may not mount properly (or something may be in the way in the quad-core case).
If you are able to compare/contrast the two cases in person…then I would do that to verify. Of course if you would be purchasing this quad-core case via the internet…this is more difficult.