compare Civ4 to Civ3?

I never bought Civ4, as I've been happy enough playing Civ3 Conquests
and I had heard that Civ4 has higher graphics requirements and also
requires XP. I was recently told that Civ4 is vastly improved over
Civ3, but have not yet heard back on my request for further details
from the person who said this.

I never play Civ3 online, only solo or in multiplayer with my kids on
our home LAN. I rarely play random maps, preferring the "Conquest"
scenarios and a couple of custom scenarios that I have made myself. I
rarely play in the modern age, preferring the Middle Ages or Age of
Discovery.

Any advice on what attractions Civ4 would have over Civ3 for me?

Thanks,

BP