According to the P3B-F's manual, the keys' position for buffered/unbuffered and 5v/3.3v are different so these buffered modules won't fit.
You probably need 3.3V Unbuffered SDRAM or EDO DIMMs. However, if you're on to SDRAM, whatever they are PC133, 100 or 66MHz rated, make sure they are made of 16-Mbit technology DRAM chips, that's a limitation of the iVX chipset. Shortly, you must get a 8-chip 16MB SDRAM, a 16-chip 32MB SDRAM or a 32-chip 64MB SDRAM (i.e, 16 chips * 16Mbits == 256Mbits or 32MBytes). No that cheap 4-chip 64-Mbit 32MB SDRAM won't fit (the iVX will recognise it as 8MB)