DDr400 with P4 2,8G FSB533 working onli at PC2100?

This is my configuration:

MB ASRock P4S55FX2 ATX AGP8x SiS655FX Soc.478
CPU INTEL P4 2.8 GHz 533MHz 512k S478 TRAY
DDR 512MB PC3200 400MHz Kingston

How can i make to work my DDR 400 to work as PC3200, now they work onli as PC 2100.

No point, your processors FSB is only 133Mhz x 4 your ram will only be effective at PC2100/133Mhz speeds, if you want your ram to run at PC3200/400Mhz, you need to be running an 800Fsb CPU, you will see no gain from running your ram faster.