P6BAT-A+ v2.0 maximum CPU and HDD supported information as below,
HDD:30G
CPU: Pentium III(SECC2). 100Mhz FSB CPU–850Mhz Pentium III(FCPGA). 100Mhz FSB CPU - 850Mhz 
           Pentium III(FCPGA). 133Mhz FSB CPU - No support Celeron (PPGA). 66Mhz FSB CPU - 533Mhz 
           Celeron (FCPGA). 66Mhz FSB CPU - 733Mhz Celeron (FCPGA). 100Mhz FSB CPU - 800Mhz
           VIA C3 100Mhz FSB CPU - 700Mhz 
           VIA C3 133Mhz FSB CPU - No support

This is old board so there is no more BIOS update for Windows XP.

The P6BAT-A+ memory speed only supports at 66MHz and 100MHz.

The latest BIOS for P6BAT-A+ V1.3A is v5.6, please link following site to click "Download -> BIOS" to get it.

Please refer to Product page - Download tag