Recent versions of MKL include em64t support for Windows x64. lapack incorporates Fortran WRITEs, so it depends on the ifort run-time library. Those functions would not work with the Portland compiler library. The MKL and compiler must both be 64-bit, or both 32-bit. Either combination will run on 64-bit Windows. The full distribution of ifort 9.x includes both compilers, just as MKL does. ifort 8.1 has a separate em64t compiler, available only by download.
Generally speaking, you would get similar performance from MKL on a dual core CPU to what you see on dual single core CPUs.
