Xfce

"Xfce is a lightweight desktop environment for UNIX-like operating systems. It aims to be fast and low on system resources, while still being visually appealing and user friendly." -from xfce website

Installation
Install the complete xfce desktop.

Or you can select only needed packages.

Autostart
To start xfce automatically with startx.

Volume keys
With xfce4-volumed daemon.

Lightweight method using alsa.

Consistent gtk3 theme
Greybird theme has support for GTK2, GTK3, xfwm4, emerald and metacity.

Go to xfce menu -> Settings -> Appearance. Or run xfce4-appearance-settings. Select "Greybird" from the "Style" list.

Now GTK2 and GTK3 applications should have consistent look.