Using high-resolution cosmological simulations, we study hydrogen andhelium gravitational cooling radiation from gas accretion by younggalaxies. We focus on the He II cooling lines, which arise from gas witha different temperature history (Tmax~105 K) thanH I line-emitting gas. We examine whether three major atomic coolinglines, H I λ1216, He II λ1640, and He II λ304, areobservable, finding that Lyα and He II λ1640 coolingemission at z=2-3 are potentially detectable with deep narrowband(R>100) imaging and/or spectroscopy from the ground. While theexpected strength of H I λ1216 cooling emission depends stronglyon the treatment of the self-shielded phase of the IGM in thesimulations, our predictions for the He II λ1640 line are morerobust, because the He II emissivity is negligible belowT~104.5 K and less sensitive to the UV background. AlthoughHe II λ1640 cooling emission is fainter than Lyα by atleast a factor of 10 and, unlike Lyα, might not be resolvedspatially with current observational facilities, it is more suitable tostudy gas accretion in the galaxy formation process because it isoptically thin and less contaminated by the recombination lines fromstar-forming galaxies. The He II λ1640 line can be used todistinguish among mechanisms for powering the so-called Lyαblobs-including gravitational cooling radiation, photoionization bystellar populations, and starburst-driven superwinds-because (1) He IIλ1640 emission is limited to very low metallicity[log(Z/Zsolar)<~-5.3] and Population III stars and (2) theblob's kinematics are probed unambiguously through the He II line width,which for cooling radiation is narrower (σ<400 kms-1) than typical wind speeds.