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>