yup..my advice is to learn photoshop to do desktop customization.
skins, icon, wallpaper etc etc are all depends on creativity using photoshop.. =p
and if u ask mr. google, he has lots of tutorial for u to grab..

hurm..yeah desktop modding sometimes can results in performance drawback..
rules of thumb: always know what u've modded, u've changed and optimize ur mod..

i always avoid using transformation pack like VTP, yeah i've used it before and i'm not satisfied at all..gave me headache with some annoying effect and changes that lower my pc performance.. besides now i think vista theme is not that pretty

and for samurize, i only make config for shortcut (as i hate shortcut icons on desktop and for my convenient) and also little bit media player info..that's all.. and set those config to use as low resource as possible, the view refresh and so on, as u dont need to refresh ur shortcut frequently, it's always the same anyway...
system monitoring config use lots of resource i think.. hehe, i tried before and sometimes caused my pc so lagging as u've to constantly refresh view to get updated graph etc etc..
yes i've compared those configs resource usage!