Which Linux distros are true 64-bit OSs?  If I buy a PC with a couple of 
X5460 chips...

http://en.wikipedia.org/wiki/Xeon#5400-series_.22Harpertown.22
http://en.wikipedia.org/wiki/List_of_Intel_Xeon_microprocessors#.22Harpertown.22_.28standard-voltage.2C_45_nm.29

...which Linux distros will have 64-bit packages ready to run on that 
architecture making full use of the available memory (e.g., 16 GB RAM)?

Mike