Be careful, though!
The original Cities XL game had some serious technical bugs, specially a memory leak that makes the game unplayable after a while. (see forum post)
The new Cities XL 2011, however, is the exact same Cities XL game, but with a bunch of bugs fixed (plus, they've added a metro system). Some people complained that old bugs are still present, and some even complain about the memory leak (but I don't know if that's fixed or not). See first impressions and list of bugs.
What's more, it seems that Cities XL 2011 game still don't run on multiple cores. However, people have a workaround, and someone has even written a tool to fix that.
Note: I don't own either game. I'm just posting because I searched the forums and decided to not buy them, so maybe this info might help you decide better.
Considering all info that I read, probably the game will run slow whatever CPU you have, as it seems the bottleneck is the CPU. (of course, that's only true if you have a decent GPU) However, maybe with enough tuning and thirdy-party tools, you might make the game run at an acceptable speed.