We study the relation between the rms mass fluctuations on 8 h^-1 Mpc scales and Ω_m 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)Ω, where γ ≡ γ(Ω_m, w), and it is valid for all w < –  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 Ω_m  0.26, w  -0.90, and σ₈  0.73, which is in excellent agreement with the recent 3 yr Wilkinson Microwave Anisotropy Probe results.