We study the relation between the rms mass fluctuations on 8 h-1 Mpc scales and Ωm using the recent clustering results of XMM-Newton soft (0.5-2 keV) X-ray sources, which have a median redshift of z ~ 1.2. The relation can be represented in the form σ8 = 0.34(±0.01)Ωimg1.gif, where γ ≡ γ(Ωmw), and it is valid for all w < – img2.gif models. By combining the X-ray clustering and SN Ia data we find that the model that best reproduces the observational data is that with Ωm sime 0.26, w sime -0.90, and σ8 sime 0.73, which is in excellent agreement with the recent 3 yr Wilkinson Microwave Anisotropy Probe results.