Do your GTK applications (i.e. Firefox) look like something designed in the 90’s in KDE? I think I can help you.
I installed the latest Firefox, (not the one in the screenshot, I stole this.) and was very disappointed to see something like the following:
Tyler pointed me to the Gentoo guide here, which helped me find out which packages I needed.
If you install Chakra-Gtk-Config, and either oxygen-gtk or qtcurve (make sure to download the gtk2 theme), you will have better looking GTK applications in no time. Note that there are probably tons of other GTK themes for KDE4, these are just some suggestions to get you started.
That is much better.