I'd want your input which gcc compiler flags to use when optimizing for Xeons?
There's no 'xeon' in mtune or march so which is the closest match?
Xeon is a marketing term, as such it covers a long list of processors with very different internals.
If you meant the newer Nehalem processors (Core i7) then this slide indicates that as of 4.3.1 gcc should be use -march=generic (though your own testing of your own app may find other settings that outperform this). The 4.3 series also added -msse4.2 if you wish to optimize that aspect of FP maths.
Here is some discussion comparing tuning in Intel's compiler versus some gcc flags.