I'm also using ubuntu 11.04, and similarly I can't significantly downclock the memory on my sapphire 5830's without either having the cards crash, or reducing the hash rate to ~150mhash/s per card. For reference, using hashkill-gpu I get around 260 mhash/s per card running at 900mem, 830 gpu. I'm going to try out some of the other utilities today.

Just added my ~750mhashes to the pool.

First of all, Nice work!

You could have shaved some cost with a cheaper CPU since you're probably not using the CPU for computations anyway, and you could run the entire thing on much less than 6GiB of RAM and just a flash drive using the linuxcoin distro ( depending on if the computer is dedicated or if you use it otherwise). How many megahashes/second are you getting with the pair of 5770's?

There is a thread in this forum that shows a number of great innovations, storing the mining rig in the snow, external PCIe extenders allowing for more cards to be added to a single system, using some "cases" which are designed for tech benches which are entirely open. There are many who are using no cases at all and pointing conventional house fans at the system boards. Make sure you check it out.

It's not feasible for me to have a system that isn't fully enclosed, I've been opting to use the Silverstone Raven case which definitely adds some expense to the system, but rotates the system board by 90deg so that the PCI cards are all at the top of the case, and then it has 3 giant fans at the bottom that blow the air up and out of the case at a low RPM which makes it significantly more quiet. I use an 1155 motherboard with enough space between the 3 PCIe slots to put 3 full sized cards in there (EVGA) and a core i3-2100t which only uses 35W. Then I tuck that badboy under my desk at work and consume the free power and air conditioning in my office. My total cost was about $1k with 3x 5830's which when tuned correctly yield 1 gigahash/second. I bought a pretty high end power supply ( Corsair 850HX ), but I'm thinking that even if the cards draw 150W each + 35 CPU + 30 Motherboard I could have probably used a 550W power supply for a fraction of the cost. I don't have a kill-a-watt to determine my actual draw.