Virtualbox: Graphics still to slow after adding 3D acceleration in Ubuntu 13.04 guest

I am running Ubuntu 13.04 both as a host and as a guest OS

My Ubuntu 13.04 host machine:
Intel Core 2 Quad Q8200 processor
Nvidia Geforce 9600 GT graphics card

My Ubuntu 13.04 guest machine running in a virtual box:
Basic Memory 512 MB
Video Memory 128 MB
3D acceleration activated

At first the graphics were very slow.
Then I added 3D acceleration (see picture below).
Now the machine is just a little bit to slow.

What can I do to improve the graphics?

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