  In this paper, we improve the previous work on the MHD Alfven wave
oscillation model for the neutron star (NS) kHz quasi-periodic oscillations
(QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS
X-ray sources with the simultaneously detected twin kHz QPO frequencies, the
stellar mass M and radius R constraints are given by means of the derived
parameter A in the model, which is associated with the averaged mass density of
star as ${\rm \lan \rho \ran = 3M/(4\pi R^{3}) \simeq 2.4\times 10^{14}
(g/cm^{3}) (A/0.7)^{2}}$, and we also compare the M-R constraints with the star
equations of states. Moreover, we also discuss the theoretical maximum kHz QPO
frequency and maximum twin peak separation, and some expectations on SAX
J1808.4-3658 are mentioned, such as its highest kHz QPO frequency $\sim$~870
(Hz), which is about 1.4-1.5 times less than those of the other known kHz QPO
sources. The estimated magnetic fields for both Z sources (about Eddington
accretion rate $\med$) and Atoll sources ($ 1% \med$) are approximately $10^9$
G and $10^8$ G respectively.
