Q:

Help locating bottleneck on new mining rig

I built a mining rig with 3 Radeon 6850 GPUs.  When I run one miner (either Phoenix or m0mchil's poclbm) it reports a hash rate of a little over 200 Mhash/sec.  When I fire up a second miner pointed at a different GPU, the hash rate of both miners is about half what the one running alone would get. When I fire up a third pointed at the last GPU, all three miners report an even lower hash rate, such that the total is around the 200 Mhash/sec that a solo miner would get.
Initially, I thought that the miners were ignoring the DEVICE=# argument and all using the same GPU. I posted a question seeking help in figuring out why they would do that. @Lodewijk's answer there clued me in to the fact that the miners are using separate GPUs and that there's a bottleneck elsewhere in the system. 
So, my question is, what's the likely bottleneck?
I used the recommendations at the bitcoin.it wiki entry on Mining Rigs to pick out my hardware.  I'm running on a Ubuntu 11.10 system, with an msi 890FXA-GD70 motherboard, 2GB Kingston 1333MHz DDR3 Non-ECC RAM, 80GB WD IDE Drive, and a 2.8Ghz AMD Semperon 145 processor. It's on a cable Internet connection with tons of bandwidth and I'm connecting to the Arsbitcoin mining pool.

A:

Did you check the CPU usage? You can use TOP on Ubuntu (sudo apt-get install top, if it's not default). If it's not capping out at 100% it's either the chipset or CPU bandwidth giving up. You can also check you regular memory, although I'm quite sure that isn't used in GPU mining.
Some things you can easily strike off are network and disk. Disk isn't used at all and network shouldn't cap out even on dialup.
If it is in fact chipset or CPU bandwidth you have two options:
1. Overclock. Focusing on FSB clock should improve everything. I'm quite sure you can't get a 300% increase though.
2. Build a different system. I daren't advice about this though.

