AMD doesn't seem to make any server CPUs that are useful for this application, unfortunately.
Really, how so? Many AMD CPU's have AES-NI. Even the A10-6800K (4 x 4.1GHz) would be decent. That plus an a85x mainboard (1Gbit) and 8GB ddr3-2133 is $300. Add some case+ps.
https://en.wikipedia.org/wiki/List_of_AMD_Accelerated_Processing_Unit_microp... https://en.wikipedia.org/wiki/List_of_AMD_FX_microprocessors#.22Vishera.22_.... https://en.wikipedia.org/wiki/List_of_AMD_Opteron_microprocessors#Piledriver...
Be careful, Intel likes to promote HT instead of full cores. There's likely some reviews of HT vs. OS schedulers out there. Intel/Opteron get pricy very quick, check price vs. performance vs. the price delta to get a second AMD node up at 2 x nMbps. A lot of the price may be in die/power and cache, which may not be of concern.