It eventually depends on the manufacturer's implementation. I think MSI and Asus were able to squeeze in the Gigabit LAN, by using a different LAN chip.
On OS, Windows7 pa rin talaga. If you go Linux, you'll not have much choice with players, except XBMC. XBMC was born in Linux I think, so maybe it would suffice? Sorry, but I don't have much visibility on the Linux world. Check sourceforge.net. I'm sure there's a wealth of players there.