For the same specs, you'll spend a lot more money on a laptop - generally almost twice as much as a desktop. So it comes down to whether the portability is worth the extra $500-700 over a desktop. Personally, I like having an awesome monitor and just replacing the tower every so often. Then again, I build computers that still blaze after 4-5 years.
You COULD be the good son and give him the new one.
Buy your dad a new low end laptop (for Christmas) and rebuild your desktop.
Remaining questions:- GPU?- What are you doing for HDDs? Using 2 PATA drives and that's it?- Why bother with the expensive CPU fan? Are you OC'ing or just like the noise reduction?