Aye, the high end ATX units do have APFC, but tend to be 4 or 5 * the price of an equivalent server unit; also the server psu's output at a nominal 12v which is more suited to my plan whereas the ATX are designed to output only a portion at 12v.
actually, with some more searching I may have answered my own question - apparently with high power PSU's there's more onus on PFC, as a 500+w psu without APFC will draw quite high peak amps at full load - hence most server PSU's, particularly the higher output ones, are APFC.
so I've taken the plunge and bought the pair I was looking at.
thanks for the replies folks