Shorting pins A1 and B17 might help you:
https://bitcointalk.org/index.php?topic=36061.0
https://bitcointalk.org/index.php?topic=369043.0

Thank you for confirming that running 8x HD79XX gpus under windows is possible.
As far as I see, most people are limited by 6 or 7 gpus. Is there any tricks to do that?

I'm also quite interested in it. There are many threads on this topic but no successful results yet.
You might need to install Linux and use some kind of virtualization to break the 8-gpu driver limit.

I'm sorry for that. I just found the link for the guide via google.
So there is no way to port the method to the latest driver?
 

Ethernet wake-up is a good method to power on, and IPMI power-on is even better, but the latter is only supported by few boards.

I'm afraid that this point is unreliable.
I know this is documented in the pcie standard, but some gpu cards seemed to not obey the standard at all and would like to pull much more than 25W from pcie slots even if they are connected through 1x extenders.

Win8 x64 supports 6 gpus but some driver tweaking might be required.
Here is a guide for it:
http://www.gobitgo.com/articles/1002/How-to-Mine-with-6-GPU/

Also worth a try to disable on-board pcie devices as much as possible in bios and see if it helps.
It's said 6 gpus did work on win7 x64 for some people, at least should be shown in device manager. See:

https://bitcointalk.org/index.php?topic=188334.0

The maximum number of gpus supported on one board also depends on the model of gpus you are running.
Since you can only see 4 cards in device manager, it might be a bios issue and not easy to fix.
I would suggest you try the method of shorting pins of A1 b17 and see if it would get more gpus to work.

It would definitely be wonderful if 20-25 gpus could work in a single system.
However, this is not easy at all. There are various limits of gpu numbers coming from bios, OS, drivers, power delivery...
Check this thread for reference:
https://bitcointalk.org/index.php?topic=64450.0

Supermicro x9drx might be an option for you, which supports up to 11 cards on a single board.
But for AMD cards you will still meet the 8-gpu driver limit.

See here:
https://bitcointalk.org/index.php?topic=365181.msg3915165#msg3915165

The product is already on the market, but looks like currently sold in several online stores in China only.
See:
http://item.jd.com/1004165.html
http://s.taobao.com/search?q=h61+btc&commend=all&ssid=s5-e&search_type=item&sourceId=tb.index&spm=1.6659421.754896237.1&initiative_id=tbindexz_20131211

Yes, it should work.
Here is a related link: https://bitcointalk.org/index.php?topic=285303.0

I would recommend for asrock h61 pro btc, which is claimed to be specifically designed for multi-gpu applications.

How about msi big bang marshal?
This one has up to 8 pcie x16 slots and is known to work for 8 cards. Current second-hand price is about 150$ in my local market.
But I also heard someone said he could only manage it to work with at most 6 cards.

Hi, rjk. It's a great pity that this interesting project was finally failed.
You should have made more progress if you had asked help from AMD directly.
I heard there was an informal "Multi-GPU" project proposed by AMD last year and finally
they got 16 GPUs (8x FirePro S10000s) to work in a single rig successfully.
So I think AMD might have already developed a proprietary driver to overcome the 8-gpu limit.
See:
http://devgurus.amd.com/thread/158863
http://fireuser.com/blog/8_amd_firepro_s10000s_16_gpus_achieve_8_tflops_real_world_double_precision_/

Did anyone give a try to supermicro x9drx?
It has 11 pcie slots and is known to be working fine with at least 18 gpus. (nvidia cards)
See https://devtalk.nvidia.com/default/topic/649542/cuda-setup-and-installation/18-gpus-in-a-single-rig-and-it-works/

Right now it's still hard for me to understand how this could be possible. I will study more about it.
Thank you!

Now I have several questions about the mining pools.
Does it need some special permissions to build a mining pool? Or everyone can build one by himself at will?