  Ultra Luminous Infra-Red Galaxies (ULIRGs) -- gas-rich mergers harboring the
most extreme star-forming conditions encountered in the local Universe -- are
thought to re-enact the galaxy formation processes we are only barely able to
glimpse in the distant Universe. Combining new single-dish molecular line
observations of 12CO, 13CO, HCO+, HCN, and CS towards the two ULIRGs Arp220 and
NGC6240 with existing data in the literature, we have compiled the most
extensive molecular line data-sets to date of such galaxies. The data allow us
to put strong constraints on the properties of the dense star forming gas in
these two systems, and compare the relative usefulness of CS, HCN and HCO+ as
tracers of dense gas. In addition, we have build molecular line templates based
on our observations, and demonstrate that Herschel/HI-FI will be able to detect
the high-J transitions of most of the above molecules in a large sample of
ULIRGs out to z<=0.5, assuming Arp220 and NGC6240 are representative of the
ULIRG population at these redshifts.
