From the Apple site:
"Mac mini computers have a single memory slot. This slot accepts a standard PC2700 or PC3200 184-pin DIMM with the following specifications:
PC2700 or PC3200 Synchronous DRAM (SDRAM)
2.5 volt
8-byte
non-parity
64-bit wide
184-pin module
maximum of 16 memory devices on the DIMM
unbuffered—do not use registered or buffered SDRAM
maximum height of 50 mm
Using PC3200 memory does not provide additional speed benefits. Although the Mac mini supports PC3200 DIMMs, the PC3200 RAM will operate at PC2700 (333 MHz) speeds."