Amazon has started accepting pre-orders for Apple's forthcoming Mac OS X 10.6, codenamed Snow Leopard, and demand is so high that the operating system has claimed the top two spots in the e-retailer's software sales charts.

The single-user version of Snow Leopard, priced at $29 (£17), is top of the charts, followed by the family version for five users costing $49 (£29). The prices are considerably lower than previous versions, which were traditionally priced at around $129 (£76).

Apple is also selling a boxed set of Snow Leopard and the iWork and iLife application suites for $169 (£100) for one user or $229 (£135) for five users. The server edition of Snow Leopard, Mac OS X Server 10.6, sells for $499 (£295).

No release date has been set for the operating system, but it is expected to be available in September. This will put it ahead of Windows 7, which is due out on 22 October.

Snow Leopard will be available only for Apple users with systems using Intel's processors, and is the first Apple operating system that uses 64-bit memory, which will make it much faster because it can use considerably more system memory.