Memory population rules on this board is kind of complex. Memory working frequency is defined by following factors:
1. Number of DIMMs on a channel. Only one DPC (DIMM Per Channel) may work at 1333MHz.
2. Organization of the DIMM. A single Quad-Rank DIMM on a channel is limited to 1066MHz.
3. The speed of the processors Integrated Memory Controller. Check your processor on http://ark.intel.com and see if 1333MHz memory is supported.
The rules above (and more) are documented in S3420GP TPS, section 3.2.4. An easier way is to download and run the Intel Memory Configuration Tool, and see what kind of memory configuration meets your requirements.