1) Necessary in linux rigs, the tools we currently have don't let us play with voltage on these cards.

2) Less hassle, bios flash lets you set it once and forget it.

~256mhash/s in my server with 2x 5830s, each at 875/1000

I've tried futzing around with AMDOverdriveCtrl but ran into stability issues with even minor tweaks.. the server they live in is the PXE host for my other miners so downtime is bad.  They also run a bit warm, ~80C, but not outside my comfort zone.

Once i get a new board to throw these in I'll be pushing them closer to 300, but stability is a necessity at the moment.

I've had really poor luck with phoenix in linux on these cards - vectors, bfi_int and worksize 128 drops me down to ~205mhash!

Tips for improving hashrate without more gpu OC in linux appreciated

This has been beat to death over and over, but get a good PSU if you plan to mine for any real length of time.  

Generally plan for 200W / card + 100W for system, and you'll have a figure to shoot for under worst-case scenarios.  Keep in mind that most PSUs won't push their full rated power output, or at least not for very long without burning out.  Research the measured power usage for the cards you plan to use, and if you think you may ever upgrade, ensure you have the capacity to support higher-end or more cards.  6990s will use ~375W each, for example.

Spend the extra to get something that's at least 80+ and 100W more than you need, and you'll likely never have a problem with it.  In the case of PSUs, the more you spend, the more you save over time.  A $100 80+ Gold supply is going to waste far less power at the wall than a $20 generic PSU; in the long run the cheap PSU is going to cost you more in utilities.

As for the other components, go as cheap as you wish long as they're well rated, and remember to leave yourself room to upgrade.  

[edit] I realize I totally glossed over the initial MB question, IMO get something that will fit you at least 4 cards, either in x16 or x1 slots, but beyond that point it becomes cumbersome to supply power to all your cards in a single system reliably!

+1 on the 6950's, I've got 4 of them running right now, stable for a couple of weeks, ~345mhash/s each without too much tweaking.

If you're really lucky, you'll get a card with unlockable shaders, and you've basically just gotten a 6970 at a discount.

Possibly marginally better power consumption from the 6990.

Figures I've seen put the 6970's at ~180-200W at full-bore (400W total), vs ~375W for a 6990.

I don't own either so this is pure speculation based on other reports.  It would make sense for the 6990 (basically two 6970's) to be a little better power-wise since they share a PCB (less overhead)

Not overly impressive or anything, but here's my three rigs (apologies for mr. blurrycam):

In action:




Ze gutz: 2 identical rigs, 2x 1gb XFX 6950 at ~345mhash/s each  (couldn't unlock the shaders   )




And my first foray into the fold, my KGPE-16 Opteron server being punished by 2x Saffire 5830s at ~250mash/s each




These guys all run with closed panels (two cats) in a closet in my basement, and they're certainly a bit warm. It's 95F ambient temp right above them (there's cooler air dragged into the intakes along the floor), avg temp for each card ranges from 80-85C.  

~1880mhash total so far

Erm, no water or bare wire all over the place?

Most computer cases are made entirely of metal, and have well worked that way for quite some time.

Just in! Top pharmaceuticals from a trusted ..  oh wait wrong forum!

Novice miner with a new cluster pushing 2Ghash/s, would love to get some tips and share my own.  I'm a programmer by trade, a programmer by hobby, and I just cannot keep my GPUs under 90C!

I'm new to bitcoin but have been pretty active in the Android community, recently contributing much to the Nook Color's custom builds.  I'd be a useful resource to those with questions about Android software, at the very least, but have decent knowledge in many IT areas, as well as several non-IT areas.  For example I just wired two new circuits into my 'server' closet, added a ventilation system, and survived the whole ordeal!  I've got experience dammit!

I also named my rigs Bebop and Rocksteady, and we all know no self-respecting spammer will admit to old school TMNT fandom.

Proof enough?  How about a helping hand?

I am not a robot!

Here's hoping the guy with 2x 6870s doesn't melt his 500W PSU before I can tell him how my similar rigs are pulling 420W at the wall! (Here, if someone else wants to warn him: http://forum.bitcoin.org/index.php?topic=21390.0)

And not to reiterate something that's likely been posted in the 30+ pages I definitely didn't read, but this restriction is kind of silly, it's got to be almost as much work to keep up with these requests as it is to blacklist the spammers..