As a SC4 owner I think performance has improved a lot since the patches on my rig. I have a region that is 1 million plus and the biggest section is 300k+, and my weak ass system can handle it. However, I will admit prior to the patch the performance is awful.
For me the game got a lot funner when
www.simtropolis.com was able to start modding the game and adding lots for new buildings in the game. If you are on the fence about the potential of this game read through that site/its forums and see what folks are doing with it. I love it now, but admit it is for those who find excruciating details fun LOL.