I thought money and research were too closely tied in Civ4. That being said, I wonder if it would have been better to have different caps on vertical expansion and horizontal expansion. In other words, gold costs for each city founded and global happiness cap just for population. That way you'd have to balance the two. It might mean large empires have more science, but they'd suffer in gold rushing (as well as ability to build things, since large cities tend to have better production). It's too late in the night for me to be thinking about this now, though.