  We have investigated 52 RXTE pointed observations of GRO J1655-40 spanning
the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months.
Our X-ray timing analyses reveal four types of QPOs: three with relatively
stable central frequencies at 300 Hz, 9 Hz, and 0.1 Hz, and a fourth that
varied over the range 14-28 Hz. The 300 Hz and 0.1 Hz QPOs appear only at the
highest observed luminosities (Lx > 0.15 Ledd), where the power-law component
dominates the X-ray spectrum. At lower luminosity, the disk flux exceeds the
power-law flux and only two of the QPOs are observed: the spectrally "soft" 9
Hz QPO, and the narrow, "hard" QPO that varies from 14-28 Hz as the hard flux
decreases. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67
Hz seen in the microquasar GRS1915+105. We discuss models of these
high-frequency QPOs which depend on effects due to general relativity. The 9 Hz
QPO displays a spectrum consistent with a thermal origin, but this frequency
does not appear to be consistent with any of the natural time scales associated
with the disk, or with the inferred values of the mass and rapid spin of the
black hole. The mechanism for the 14-28 Hz QPOs appears to be linked to the
power-law component, as do the 1-10 Hz QPOs in GRS1915+105. Finally, we show
data for GRO J1655-40 and GRS1915+105 as each source teeters between relative
stability and a state of intense oscillations at 0.1 Hz. A comparison of the
sources' spectral parameters allows us to speculate that the black hole mass in
GRS1915+105 is very large, possibly in the range 39-70 Msun.
