Note I also posted about this in the Berkeley Forums.

Having problems with an AMD/ATI driver install.
Pulled a GTX470 and installed an HD 5850.

Tried the auto detect AMD install but had to chose XPx64, while using a Server (2003x86). After rebooting there was no indication that the drivers took, so I uninstalled all NVidia ware, rebooted and did a manual install of the driver (just the driver). While the driver is there, Boinc does not see the GPU and reports, Caught SIGSEGV in ATI GPU detection:

06/08/2011 00:13:20 | | Starting BOINC client version 6.12.33 for windows_x86_64
06/08/2011 00:13:20 | | Config: report completed tasks immediately
06/08/2011 00:13:20 | | Config: use all coprocessors
06/08/2011 00:13:20 | | Config: zero long-term debts on startup
06/08/2011 00:13:20 | | log flags: file_xfer, sched_ops, task, coproc_debug, cpu_sched_debug, dcf_debug
06/08/2011 00:13:20 | | log flags: debt_debug, rr_simulation, sched_op_debug, work_fetch_debug
06/08/2011 00:13:20 | | Libraries: libcurl/7.19.7 OpenSSL/0.9.8l zlib/1.2.5
06/08/2011 00:13:20 | | Data directory: E:\BOINC
06/08/2011 00:13:20 | | Running under account Administrator
06/08/2011 00:13:20 | | Processor: 8 GenuineIntel Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz [Family 6 Model 42 Stepping 7]
06/08/2011 00:13:20 | | Processor: 256.00 KB cache
06/08/2011 00:13:20 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 cx16 sse4_1 sse4_2 syscall nx lm vmx smx tm2 popcnt aes pbe
06/08/2011 00:13:20 | | OS: Microsoft Windows Server 2003 "R2": Standard Server x64 Edition, Service Pack 2, (05.02.3790.00)
06/08/2011 00:13:20 | | Memory: 7.98 GB physical, 23.53 GB virtual
06/08/2011 00:13:20 | | Disk: 931.51 GB total, 69.96 GB free
06/08/2011 00:13:20 | | Local time is UTC +1 hours
06/08/2011 00:13:20 | | No NVIDIA library found
06/08/2011 00:13:20 | | Caught SIGSEGV in ATI GPU detection
06/08/2011 00:13:20 | | No usable GPUs found

Any suggestions?

Which driver(version and type) did you installed?

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I tried several drivers (present 11.7, early versions and beta 11.8), each time with a clean install. Went back to Boinc 6.10.60 and tried several more.
Basically I got nowhere, and was going round in circles. About 30 reboots is my limit. I think it's fair to say that ATI/AMD simply don't support a 2003 x64 server. It worked fine on XP x86, but on 2003 the drivers don't seem to report to Boinc correctly. I can get an all clear on Device Manager and GPUZ reports the GPU correctly, but Boinc say's no. Perhaps the drivers did not register correctly in some way. I've spent too much time on this so I'm giving up. Might find another host at some stage, but for now I'm out of here.

I remember having to to install a special ATI driver for Milkyway, so I didn't need to install the ATI SDK. So, is there just one ATI driver now?

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According to the most recent SDK release, XP support is deprecated and being dropped in the next one, so things are probably going to start breaking.