maybe i'm too late (the original poster may have bought civ3 already), but i'd like to summarize a few points about the two games:
- both have the same addictive qualities
- civ3 has borders
- civ2's zone of control and unit hitpoints are gone. {snip}
- now civs have different traits, and this was a good addition to the game {snip}
- strategic resources are one of the best features added to civ3. {snip}
- now certain city improvements and wonders generate culture, and this creates another victory condition in the game - the one with the most culture wins.
- civ3 AI is superior to the one in civ2, {snip}
- diplomacy options were expanded and diplomatic negotiations are now even more important than before. {snip}
- in civ3 your units are not supported by individual cities, but by your budget. this makes it easier to maintain a large army.
- when civ3 was first released, it had a very disappointing and limited editor, but the expansion packs and numerous patches improved this situation and now you have good scenario capabilities.
- no wonder movies in civ3
of course, there are other differences between both games, but i think these ones i cited can give you a good overview of how civ3 compares to civ2.
sorry for the long post.