We present radial mass profiles for 16 relaxed galaxy groups (kT=1–3 keV) selected for optimal mass constraints from the Chandra and XMM data archives. The resulting mass profiles are described well by a two component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. For the first time we find that the NFW concentration parameter (c) for groups decreases with increasing virial mass (M) as expected in standard Lambda-CDM models. When combined with our own results for 7 elliptical galaxies and clusters from the literature, the X-ray c-M relation agrees with the relation produced by the flat, concordance Lambda CDM model provided the sample is comprised of the most relaxed, early forming systems, which is consistent with our selection criteria. The tilted, low sigma_8 model suggested by the 3-yr WMAP analysis is rejected at > 99.99% confidence, but it can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w = -0.8.