Me and a friend are building a rig, 6990x3, pretty much as listed on the wiki mining rigs page. ( https://en.bitcoin.it/wiki/Mining_rig#Three_ATI_6990s,_Approximately_2.1_Ghash/s
1. I have a home office and would like to use some basic spreadsheets and at least play some SC2 and some Mass Effect (1/2/3) on it. How does that affect mining? I've read a post on multitasking (gaming) while mining but I'm not clear on whether the GPGPU ceases to function completely or if it just slows down.
2. Also I'm not clear on how much power is required. It seems like 150W for the PC + components and 150W per 6990. So about 600W. In your experience is this enough?
3. How do 3 cards behave in one machine?
3.1 Are there MAJOR cooling issues or does leaving the case open solve the problem?
3.2 Are there space issues?
3.3 Can another card fit? (another friends wants to know if he can come in on the action and add a 4th 6990)
4. Form what I've read, Windows and Linux perform pretty much on par so there is no big reason to choose Linux over Windows, is this accurate?
5. Are there any major unforeseens that experience has taught you with this kind of rig?
1. not sure about the 4 gpu limit in Windows (I think Windows actually supports up to 8 gpu's, the 4 gpu limit might be just for crossfire), but if you want to use the machine while mining, you can just set the miners on your primary video card (the one your monitor is hooked up to) to lower aggression. That will reduce the gui lag and at low enough aggression even allow you to do some gaming (though that will reduce your hashrate on that video card - some from lower aggression, some from utilizing the gpu for game rendering).
There is one issue though - for mining on the 6990's you will have to run the 2.4 version of AMD SDK. Under Windows, that will utilize 100% of a cpu core per miner (gpu). With two 6990's, that will means 100% utilization of a 4-core cpu. That will increase power consumption and reduce the machine's responsiveness. This doesn't occur under linux.
2. a 6990 under load uses some 300-350W. With overclocking that can reach 400W. Add some 200W for the rest of the system, less if you build a dedicated mining rig. For 3 6990's, you'll need at least a 1200W psu, more if you plan to overclock.
3.1. yes, removing >1kW of heat from a case is a MAJOR issue. Just removing the side panels on the case will not do, you will also have to ensure very good airflow through the case (like several high-airflow case fans). You will still probably have to run the fans on the 6990's at max or near-max rpm, and they are really LOUD. As in, three of them will sound pretty much like a vacuum cleaner.
3.2. Make sure the pci-express slots on your motherboard are spaced far enough apart to allow installation of dual-slot video cards. Even then you will probably have to use spacers between the cards to ensure enough separation for adequate airflow.
3.3. There are motherboards that can accommodate 4 dual-slot video cards. In that case you can theoretically add another 6990, but that will further increase problems with heat, noise and power requirements.
5. don't even think about running a rig with >1kW of power consumption in a room without air conditioning