I've got Athlon64.
Built the latest CVS version of boinc, then SETI.
Server refuses to accept platform 'x86_64-unknown-linux-gnu'.
Ok. Then i used i686 build of boinc along with x86_64 build of SETI + app_info.xml
It started to work, downloaded a result and started calculating. It lasted for 12 hours:
-------------------------------------
2005-10-21 01:30:12 [SETI@home] Started download of 11mr04ab.25869.8144.534648.227
2005-10-21 01:30:19 [SETI@home] Finished download of 11mr04ab.25869.8144.534648.227
2005-10-21 01:30:19 [SETI@home] Throughput 51445 bytes/sec
2005-10-21 01:30:19 [---] request_reschedule_cpus: files downloaded
2005-10-21 01:30:19 [---] schedule_cpus: must schedule
2005-10-21 02:30:20 [---] schedule_cpus: time 3600.096595
2005-10-21 03:30:20 [---] schedule_cpus: time 3600.079977
2005-10-21 04:30:20 [---] schedule_cpus: time 3600.080006
2005-10-21 05:30:20 [---] schedule_cpus: time 3600.079994
2005-10-21 06:30:20 [---] schedule_cpus: time 3600.080013
2005-10-21 07:30:20 [---] schedule_cpus: time 3600.079994
2005-10-21 08:30:20 [---] schedule_cpus: time 3600.080000
2005-10-21 09:30:20 [---] schedule_cpus: time 3600.080012
2005-10-21 10:30:20 [---] schedule_cpus: time 3600.079987
2005-10-21 11:30:20 [---] schedule_cpus: time 3600.080836
2005-10-21 12:30:20 [---] schedule_cpus: time 3600.079178
2005-10-21 13:12:02 [---] request_reschedule_cpus: process exited
2005-10-21 13:12:02 [SETI@home] Computation for result 07my04aa.12165.12784.528386.157_2 finished
-----------------
I think there is some error, previous i686 version worked much faster, and it looks like that x86_64's version results was corrupted or even not finished at all, they didn't appear on my account page as FINISHED.

P.S.
x86_64 version of BOINC shows these results:
Measured floating point speed 1464.65 million ops/sec
Measured integer speed 3821.94 million ops/sec

and i686:
Measured floating point speed 1001.43 million ops/sec
Measured integer speed 2138.43 million ops/sec

64bit is almost twice faster.

---

....
2005-10-21 11:30:20 [---] schedule_cpus: time 3600.080836
2005-10-21 12:30:20 [---] schedule_cpus: time 3600.079178
2005-10-21 13:12:02 [---] request_reschedule_cpus: process exited
2005-10-21 13:12:02 [SETI@home] Computation for result 07my04aa.12165.12784.528386.157_2 finished
.......
here is some error, previous i686 version worked much faster, and it looks like that x86_64's version results was corrupted or even not finished at all, they didn't appear on my account page as FINISHED.

If I understand your problem right, your WU will running for a very long time ?

At http://www.lb.shuttle.de/apastron/boincDown.shtml#gnua64 you will find a stable and good optimized boinc client and seti app for Linux on AMD64.

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