I have a lot of stuff installed on my N900 (kernel power 40, multiboot 0.2.9, nitdroid flying world, sygic mobile maps, psx4all, conky, cuteexplorer, fm radio, recorder, queen beecon widget etc.) and my phone is smoothness switching from one to another (i have also many widget on desktop).

Actually, the only way to obtain smoothness is overclock.

Mine is set to ideal (500-850) and my theme is R-Type Blue.