I disagree. I think 512MB will be more than enough for your dad if its his first time using a computer.
basically, id imagine all he'll ever be doing is surfing the web, and maybe downloading photos. Even the cheapest laptop you can find will be suitable - so i would spend any more on getting more RAM etc - it'll be wasted.
FYI - im running a laptop with 1gig RAM, using Vista (very thirsty). It runs perfectly fine, and im a heavy user - dvd's, music, photoshop etc etc. The figures now are more than ever will be needed - they just exist to sell units.