With three weeks left before the promised ship date of OS X Leopard, the long-awaited and much-delayed sixth major update of Apple’s (AAPL) flagship Macintosh operating system, Piper Jaffray’s Gene Munster is already calculating its impact on the company’s revenue stream.

In a note to clients issued this morning, Munster observes that OS X Tiger, Leopard’s predecessor, was also released at the end of the first month of a fiscal quarter (April 29, 2005 vs. Oct. 26, 2007). He writes:

At that time, the OS X installed base was 12 million and Tiger sales added $125 million to the quarter. The Mac OS X installed base is now approximately 23 million, so we expect Leopard to add approximately $240 million to the Dec. 2007 quarter. This assumes similar uptake rates to the Tiger launch, which saw 15% of the user base upgrade in just 6 weeks (eventually 66% of the user base upgraded to Tiger).

Looking ahead to the next Macworld, Steve Jobs’ favorite venue for announcing new products, Munster anticipates one of two possiblities:

• a multi-touch PDA slightly larger than an iPhone (which some are calling the new Newton)
• an ultra-portable Mac that’s smaller than the smallest MacBook (which AppleInsider has dubbed the ThinBook)

“If Apple launches a new product at MacWorld in January,” Munster writes, “we believe it will likely fall into one of these two categories.”