Though installatioin of SMF is described in
another topic I can tell you some things
to do before installation, to modify your forum and install light.
For example - you can delete every other theme besides default theme in Themes folder. You can delete classic smileys, since they suck. You can replace old avatars
with yours which you can get on livejournal - 100x100.
Additionally, you can even replace default theme with some other theme, by replacing theme files and images. Basically - with SMF you can do whatever you want.