What's the reccomended distance betwen cities?

Well cities can work the land 3 spaces away from them, so I guess leaving 6 spaces between cities is optimal, do stop them clashing. That's what I try and go for, anyway.

A city can rarely get to the point where it will be working all it's tiles, for example a usual size for a city is what 10-15 even the biggest cities of 20-30 pop will not be working all their tiles, since by that tie they will have specialist buildings. Due to that fact that it is easier to defend closer cities and also because roads cost money the general consensus seems to be that the distance of about 4 squares is sufficient. The more important factors are obviously terrain and resource placements. It's more important to build cities to better utilise the natural resources then to go by distance, so when you need to it's no biggie to build cities 3 tiles away or 10 tiles away.

I mainly choose my city locations just to add luxury resources to my empire.  
 
Other than that, I just want cities that have a few high-performing tiles. Since working them is restricted by population, I only need a handful of really good tiles, and the average tiles aren't going to get used.