what is more important ?

Hiya ,

Just wanted some views/opinions on what is the most important piece of equipment in the reproduction of sound/music.

I know and have heard that the dimensions of the room has the biggest impact but what about the speakers, amplification, and source... which one would give the biggest sonic differences(for better or worst) if replaced ?
If my current system needs upgrading which part of the chain should I focus on first to get the best 'bang for my bucks' ?

Cheers all