  In this paper we explore for the first time the relative magnitudes of three
fundamental sources of uncertainty, namely, foreground contamination, thermal
noise and sample variance in detecting the HI power spectrum from the Epoch of
Reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis
telescope to detect EoR based on its array configuration and a statistical
representation of images made by the instrument. We use the Murchison Widefield
Array (MWA) configuration for our studies. Using a unified framework for
estimating signal and noise components in the HI power spectrum, we derive an
expression for and estimate the contamination from extragalactic point-like
sources in three-dimensional k-space. Sensitivity for EoR HI power spectrum
detection is estimated for different observing modes with MWA. With 1000 hours
of observing on a single field using the 128-tile MWA, EoR detection is
feasible (S/N > 1 for $k\lesssim 0.8$ Mpc$^{-1}$). Bandpass shaping and
refinements to the EoR window are found to be effective in containing
foreground contamination, which makes the instrument tolerant to imaging
errors. We find that for a given observing time, observing many independent
fields of view does not offer an advantage over a single field observation when
thermal noise dominates over other uncertainties in the derived power spectrum.
