With Microsoft Windows 7 in its rearview mirror, Apple is making Snow Leopard an inexpensive operating system upgrade for Mac users.

Philip Schiller, senior VP of worldwide marketing, told attendees at Apple’s Worldwide Developers Conference in San Francisco Monday that for $29 they can upgrade from the current Mac OS X, Leopard. The new version boasts better performance and a number of enhancements, most notably delivery of a full 64-bit kernel. Apple also is offering a “family pack” of up to five Snow Leopard licenses for $49.

Schiller also announced September availability of the new OS, a month before Microsoft releases Windows 7. Analysts have been impressed with Microsoft’s follow-up to Windows Vista and say Windows 7 is likely to present the greatest challenge to Mac OS X.

In taking a dig at Snow Leopard’s rival, Bertrand Serlet, senior VP of Mac OS X for Apple, tried to link Windows 7 to its much maligned predecessor Vista. “Underlying Windows 7, you still have the old technologies [of Vista],” he said.

However, analysts and reviewers say Windows 7 is actually a big improvement over Vista, and offers a user interface that’s comparable in ease of use to Mac OS X. If Windows 7 proves popular with consumers, then it could convince many people not to switch to a Mac.

Much of Snow Leopard’s performance boost can be attributed to the move to a 64-bit kernel, which offers support for a virtually unlimited amount of system memory. In addition, applications written for the new architecture will run faster. Apple’s own applications will run natively on the 64-bit Snow Leopard.

Other enhancements include an Apple technology called Grand Central Dispatch, which makes it possible to schedule processes across all available CPU cores in parallel. Such a feature is important with the use of Intel’s multicore processors in Macs.

Apple also is making OpenCL available to allow developers to tap into the processing power of graphics processors when they’re lying idle. OpenCL, or Open Computing Language, is a framework for writing programs that execute across CPUs, graphics chips, and other processors.

Other improvements in Snow Leopard include support for Exchange Server 2007, Microsoft’s messaging software, and the introduction of QuickTime X, Apple’s next-generation platform for running video and other media content.

Support for Exchange makes Snow Leopard friendlier to businesses. While its unlikely companies will toss Windows PCs in favor of Mac, Exchange support will be a major plus for employees who prefer MacBooks.