  We compute the leading low-energy term in the planar part of the 2-loop
contribution to the effective action of $\N=4$ SYM theory in 4 dimensions,
assuming that the gauge group $SU(N+1)$ is broken to $SU(N) x U(1)$ by a
constant scalar background $X$. While the leading 1-loop correction is the
familiar $c_1 F^4/|X|^4$ term, the 2-loop expression starts with $c_2
F^6/|X|^8$. The 1-loop constant $c_1$ is known to be equal to the coefficient
of the $F^4$ term in the Born-Infeld action for a probe D3-brane separated by
distance $|X|$ from a large number $N$ of coincident D3-branes. We show that
the same is true also for the 2-loop constant $c_2$: it matches the coefficient
of the $F^6$ term in the D3-brane probe action. In the context of the AdS/CFT
correspondence, this agreement suggests a non-renormalization of the
coefficient of the $F^6$ term beyond two loops. Thus the result of
hep-th/9706072 about the agreement between the $v^6$ term in the D0-brane
supergravity interaction potential and the corresponding 2-loop term in the 1+0
dimensional reduction of $\N=4$ SYM theory has indeed a direct generalization
to 1+3 dimensions, as conjectured earlier in hep-th/9709087. We also discuss
the issue of gauge theory -- supergravity correspondence for higher order
($F^8$, etc.) terms.
