K7S5A v3.x maximum CPU and HDD supported information as below,
HDD:250G
CPU: AMD Duron 200Mhz FSB CPU - 950 AMD Duron 266Mhz FSB CPU - 1800
           AMD Morgan 200Mhz FSB CPU - 1.3G 
           AMD Athlon 200Mhz FSB CPU - 1.4G AMD Athlon 266Mhz FSB CPU - 1.4G
           AMD Palomino 266Mhz FSB CPU - 1.2G
           AMD Athlon XP (0.18u) 266Mhz FSB CPU - 2100+ AMD Athlon XP (0.13u) 266Mhz FSB CPU - 2600+

No, please don't  mix SDARM and DDR memory modules at the same time. Otherwise, the motherboard may be damaged.

Please follow below steps to set up "Wake-On-LAN function" on K7S5A V3.1B.
(1) Enable "PowerOn by LAN/Ring " in BIOS\Power management setup.
(2) Both "Ring in and PME 2" are supported by WOL function. If you intend to wake on LAN by ring, please plug the ring-in line which connect with LAN card in WOL 3 pins junction on the motherboard.
(3) Please note that "WOL function" of K7S5A works only when system shutdown is performed under OS. This feature does not work after you press the power button over 4 seconds to power off the system.

Please check if your motherboard's battery works well or not. If your battery capacity is low, the CMOS data can't be kept.

No, the K7S5A does not support AMD Sempron CPU.

The K7S5A can support to AMD AthlonXP 2600+ FSB 266MHz CPU.

This model support DDR 266 module RAM then check your PCB version to update the latest BIOS on ECS website. If you use DDR RAM please remove SD-RAM. Don't use together.