CPU for moderatly heavy Google Sketchup models

Manipulation of models in a 3D viewport is dependent on the video card. If details are disappearing when rotating a view then your card is not powerful enough to keep up with the redraws. A good modern game card will help a lot. Better yet, a good workstation card like a FirePro or a Quadro will be a huge improvement. Concerning your CPU I do recommend the i7 over the i5 for 3D work, but your current issue with slow redraws is due to your video card.

Sketchup is a single core application. One that with more complex models will give you endless problems and extremely poor performance. So basically get the absolutely fastest CPU. If it is a single core CPU that is fine. Hell, perhaps a P4 makes more sense! All the extra CPUs that a i5 or i7 offer are wasted, unless you multitask. Only exception is while rendering, then yes, if you use a modern renderer it will be faster.

In any case, Sketchup is good for drafting, but horrible once you add in 3ds models or have many polygons/faces in the scene. It will freeze, lockup for 5-50 minutes during opening and saves, it will corrupt your scene, it will give you all sorts of trouble.

If you want the ease of use of Sketchup but good performance try Cinema4D (an identical scene will not give any trouble at all in C4D, while it locks up for 5 minutes for basically everything you do on Sketchup, same with other proper 3D programs, they won't give you problems). You want to suffer, 3ds max.