Far be it from me to talk you out of a purchase, however the 1.4 Athlon is less than 10% improved on the 1.333 performance (according to Tom), so unless you are coming from something less than 900Mhz I don`t really think a 1.4 will show much gain for your buck!.
However it is your buck so it is your say!.
About the support on a ASUS A7M266, you should have little problem with support and as you suggest a BIOS upgrade is the way to go.
My only qustion would be, "do you have the A7M266, or are you purchasing this too?".
On the Edge of Extremity
Microsoft Registered OEM,DSP,MCP
Custom Building of PC`s.