Based off of the Windows app_info posted in the Moo section of the project forums, I created one for my Linux box to force x86_64 instead of i686. Adjust as necessary for your particular setup. I reserved a CPU core for the app, just in case. Linux doesn't seem to handle fractions of a core very well. I'll remove the requirement if it's apparent there's no performance increase by reserving the entire core.

Code:
<app_info>
  <app>
    <name>dnetc</name>
    <user_friendly_name>Distributed.net Client</user_friendly_name>
  </app>
  <file_info>
    <name>dnetc_wrapper_1.3_x86_64-pc-linux-gnu__ati14</name>
    <executable/>
  </file_info>
  <file_info>
    <name>dnetc518-linux-amd64-stream</name>
    <executable/>
  </file_info>
  <file_info>
    <name>dnetc-gpu-1.3.ini</name>
  </file_info>
  <file_info>
    <name>job-lin64-ati14-1.00.xml</name>
  </file_info>
<app_version>
    <app_name>dnetc</app_name>
    <version_num>103</version_num>
    <platform>x86_64-pc-linux-gnu</platform>
    <avg_ncpus>1.000000</avg_ncpus>
    <max_ncpus>1.000000</max_ncpus>
    <flops>133552695568.405975</flops>
    <plan_class>ati14</plan_class>
    <api_version>6.13.12</api_version>
    <file_ref>
        <file_name>dnetc_wrapper_1.3_x86_64-pc-linux-gnu__ati14</file_name>
        <main_program/>
    </file_ref>
    <file_ref>
        <file_name>dnetc518-linux-amd64-stream</file_name>
        <copy_file/>
    </file_ref>
    <file_ref>
        <file_name>dnetc-gpu-1.3.ini</file_name>
        <open_name>dnetc.ini</open_name>
        <copy_file/>
    </file_ref>
    <file_ref>
        <file_name>job-lin64-ati14-1.00.xml</file_name>
        <open_name>job.xml</open_name>
        <copy_file/>
    </file_ref>
    <coproc>
        <type>ATI</type>
        <count>1.000000</count>
    </coproc>
    <gpu_ram>262144000.000000</gpu_ram>
</app_version>
</app_info>