I'm not sure enforcing sse3 or later for AMD64 hardware is the best
course of action.
> egrep "^model name|^flags" /proc/cpuinfo
model name : AMD Athlon(tm) 64 Processor 3000+
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext fxsr_opt lm
3dnowext 3dnow rep_good nopl eagerfpu pni lahf_lm 3dnowprefetch vmmcall
This machine is still able to run Arch just fine, and has saved the day
when the laptop I used day to day broke down. As others have said, AMD
cpus might not have sse4 unless they are quite recent.
Requiring sse2 might be a reasonable cutoff, but I'm sure there will be
people still using older machines that will complain if sse2 is
mandatory, as I can reasonably see late pentium 3 machines still being
usable for office, email or light browsing applications.