Or using a multi argument Dynamic like this
DynamicModule[{f, x, arg, val}, f[x_] := {x, Sqrt[x]}; x = 0; {arg, val} = f[x]; Column[ {Slider[ Dynamic[x, ( x = #; {arg, val} = f[x] ) & ], {0, 25, 1} ], Row[{"{arg,val} = ", Dynamic@{arg, val}}]}] ]
This works. Thanks David.