<p>I have to implement MPI system in a cluster. If anyone here has any experience with MPI (MPICH/OpenMPI), I'd like to know which is better and how the performance can be boosted on  a cluster of x86_64 boxes.</p>

MPICH has been around a lot longer.  It's extremely portable and you'll find years worth of tips and tricks online.  It's a safe bet and it's probably compatible with more MPI programs out there.
OpenMPI is newer.  While it's not quite as portable, it supports the most common platforms really well.  Most people seem to think it's a lot better in several regards, especially for fault-tolerance - but to take advantage of this you may have to use some of its special features that aren't part of the MPI standard.
As for performance, it depends a lot on the application; it's hard to give general advice.  You should post a specific question about the type of calculation you want to run, the number of nodes, and the type of hardware - including what type of network hardware you're using.