If you're getting the board from a friend or eBay (cheap) all I can say is put a stick in there and try it, worst case scenario it won't boot (assuming your CPU is supported). As you already know the P5KPL AM-PS does not officially support 533 sticks. If you're buying a new board might want to go with something that officially supports the ram. The ASUS P5N-D is inexpensive, it supports 533 modules and P4-3.20F GHz (EM64T), P4-3.40F GHz (EM64T), P4-3.60F GHz (EM64T), and P4-3.80F GHz (EM64T) processors.
P5KPL AM-PS Memory Specs:
-Front side bus 1600 (O.C.)/1333/1066/800 MHz
-Dual channel memory architecture 2 x 240-pin DIMM sockets support unbuffered non-ECC 1066 (O.C.)/800/667 MHz DDR2 memory modules.