I've started looking into what it will take to upgrade the processor in the HP EX470/475. The stock processor is the AMD Sempron 3400+, but AMD makes several voltage configurations (see this comparison chart from AMD's web site). I haven't verified if the 3600+ or 3800+ will work in the EX470 yet.
It would appear as though the processor in the EX470 is AMD part # SDD3400IAA3CN. It is likely that a Sempron 3500+ will work (see this comparison chart) and while you gain 200 MHz in speed, you loose 128MB in L2 cache. The Athlon 64 X2 Dual Core has a different voltage listed. The Athlon 64 3500+ looks like a winner. The 3600+ and 3800+ (mentioned above) will NOT work as they are not 35W parts.
Below is a screen showing the output from CPU-Z. It shows an AM2 (940) socket. I'll keep you posted as to my findings.
It has been pointed out that the Core Speed in the above screen shot is too low (it should be 1800 MHz not 1000 MHz). It appears that the AMD CPU has a mode called "Cool n Quiet" that reduces the core speed during periods of low CPU utilization.
I've taken another screen shot during a file search:
WHS, Windows Home Server, HP, HP Server, MediaSmart Server, HP EX470, Processor, CPUs, AMD, Upgradtion, Hardware, Tips and Tricks
Source:? Home Server