You can switch between the visual-style (taskbar, start-menu, window borders, etc.) but the icons will stay Vista-style.
If you install the Bricopack in safemode, you shouldn't have a problem (Google for instructions on how to get into safemode).
If you disable RocketDock, Y'z Shadow, UberIcon and TransBar (which provide some extra Vista-like effects), you should not suffer from decreased performance.. 