Since I'm using 4 flatscreens this dual GPU solution from Gecube seemed to be the optimal choice for me. It enables the user to output to 4 DVI devices (analog with dongle) and delivers enough 3D power for current games. In theory, the card has been tailored for me, but the reality is a little bit different.

UPDATE: Issue fixed!

So, let's start off:

Noise
The first thing that really disturbed me when switching on my computer was the noise of the cooler of the card. This humming is really causing pain when you listen to it for a few minutes and I'd nearly called Boeing, if they are missing a turbine. So if you plan to use the card for a workstation you might be advised to stick with a dual card solution with passive cooling. I was hoping that the fan is temperature sensitive, but it isn't, it's always running at maximum RPM.

Driver
After installing the ATI Catalyst drivers for Windows XP x64 on my system (Intel Core 2 Quad Q6600 on a Intel DP35DP mainboard), I felt that the system performed really poor in 2D mode. I took a look at the taskmanager and noticed a couple of processes running at 25% CPU (100% per core). Starting Firefox took (and still takes) minutes, browsing is impossible. It is not predictable which programs and processes are affected of this injection of "Handbrake 3.0 Extreme Edition".

I contacted ATI support and after two standard responses constructed of templates, they tried to solve my problem, but could not offer any measures which I hadn't already tried (reinstalling chipsetdrivers etc). It ended up with the final conclusion of the support team, that they cannot reproduce the problem, the card was tested with the chipset and seems to work. They told me, that I should test the card in another system, to see if the problem follows the card. However, if the card works in another system it is no solution for me, because I have to get this combination of hardware working. Under Vista x64 the situation is a bit different. The drivers seem to work fine, but the above mentioned problem still occures randomly.

I also contacted Gecube support, but I received no response until now, which I consider definitly as evidence of incapacity.

Conclusion
The concept of the card is really nice and seemed to be perfect for me. The noise is inacceptable for a workstation. The driver situation is really annoying, because it is not possible to construct a stable system. I spent days experimenting and was not able to find a solution. I hope that others will have that problem too, so that ATI is forced to fix the driver (see update).

UPDATE@071030: The reason for this problem may have been a brick of memory which dismissed with a buzz today. Unfortunately I've tested for memory errors several times, but the test results were 100% ok. I'll keep you updated, if the situation changes with the new memory.

UPDATE@071105: I replaced the broken kit memory with 2 new bricks à 1GB today and had the same problem again. It seems now, that the problem only occures with 4GB of system ram. With 2 or 3 GB everything is working fine. I informed ATI about the new aspects of this issue and I'm curious about the response.

UPDATE@071218: The story continues. I am still having problems with the same issue. ATI support told me that they cannot reproduce the problem and closed the support request.
Today tried a NVIDIA GeForce PCX 5300 graphics card manufactured by Gigabyte and everything worked fine with the same configuration (4GB). This leads to the conclusion that the problem is either caused by the ATI driver or the Gemini 3 itself. I contacted Gecube support again, I hope they even respond this time!

UPDATE@080127: Issue seems to be history. Intel fixed the Vista x64 4GB bug in its newest version of the Bios for the DP35DP board, which obviously has something to do with my problem. The system is performing fine now, I'm happy.
PS: I still have no response from Gecube.

UPDATE@080421: Intel has released several versions of the mainboard BIOS until today. One of them reimplanted the bug again, but reverting back to the previous release was a solution. With the current version of the BIOS the system is fast and stable.
PS: Gecube support sucks.