PC wont post after adding another 512MB stick

[Vanilla]

Hi all, i'm probably doing something wrong!

I have a Gigabyte K8. In this I have four memory slots. In the first two memory slots I have a pair of dual channel 512mb Geil sticks. I have just bought a third stick to give a bit of extra grunt but if I put it into slots 3 or 4 the PC will not post (turns on but no picture on the monitor).

So:

Slot 1: Paired Geil 512mb stick
Slot 2: Paired Geil 512mb stick
Slot 3: Wont post if new stick here!
Slot 4: Wont post if new stick here!

Any idea what I have to do to get it working?

 

[Vanilla]

Bah, my mobo doesn't support 3 memory modules!

 

[xija06]

I'm not sure if you can run dual channel sticks and a single channel together? Might be wrong though!