I have a 370DLE motherboard, currently runs with Intel Pentium III 370FC-FGA 866MHz, can I upgrade CPU to 1.26GHz processor?
No, the 370DLE motherboard only supports up to 1.0GHz PIII 370FC-PGA Coppermine processor. The Intel PIII 370FC-PGA2 1.13GHz and above speed are released as Tualatin processors family and they won't work on 370DLE motherboard or any other Supermicro's boards belong to 370 series family. If you want to use Tualatin CPUs up to 1.4GHz you must use any board belong to P3T series.