Apple has released its next operating system for pre-order in the States, just a couple of weeks after Microsoft did the same for its forthcoming OS.

Apple's Snow Leopard OS X 10.6 and Microsoft's Windows 7 will go almost head-to-head with September and October releases respectively.

On, Snow Leopard is currently the number one best seller in the software category, taking up both the first slot, for the $29 single upgrade version of the OS and the second slot, for the $49, five-user, "family pack". Windows 7 does not feature in the top ten.

Amazon states of the pre-order process: "Please note: Official release date has not been announced by Apple, though they have indicated this product will be released sometime in September".

There is no such pre-order in place on Amazon in the UK, where pricing for Snow Leopard is yet to be set, and Apple's official site only offers email notification when the OS is closer to availability.