650Mhz is plenty for 90% of the available skins. Just stay away from the Flash skins and you're fine. Many of the newer skins also include flash addons, but they can be disabled or removed without losing much. As I have learned over the years, you need to focus more on what's taking up memory space and then what's using the CPU. I run flash skins (slowly), but fully functional on my 500Mhz Tablet.
So don't worry about small skin, pick whichever one you like. That's more important - and keep tweaking the system.