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, the K7S5A does not support AMD Sempron CPU.

The following website is the CPU support table that ECS had tested and qualified for use with this motherboard.

http://www.ecs.com.tw/ECSWebSite/Product/Product_Detail.aspx?DetailID=192&CategoryID=1&MenuID=50&LanID=0#fragment-CPU