By Slash Lane

Published: 01:19 PM EST (10:19 AM PST)

The release of new MacBook Airs and the Mac OS X Lion operating system last month helped spur double-digit growth for Apple, which is now estimated to sell a record 4.5 million systems during the ongoing September quarter.

Preliminary figures for the PC sales released by market research firm NPD on Monday indicate that sales of Macs in the United States rose 26% year-over-year in July, which puts the Mac maker on a pace to meet consensus estimates of 4.5 million total sales for the three-month period ending September.

Gene Munster, an analyst with Piper Jaffray, attributed the rise to new Mac products but warned that it will be more challenging for Apple to maintain such growth throughout the quarter, as the company saw 29% and 20% respective growth rates during August and September of 2010, compared to July's 14%.

"We believe Mac sales benefited from the 7/20 launch of OS X Lion along with refreshed MacBook Airs and Mac minis," he said. "But these tailwinds will fade throughout the September quarter and year-over-year compares get slightly tougher in the last two months of the quarter."

Performing his own extrapolation and analysis of the NPD data, Munster estimates that Apple is pacing to sell 4.4 million to 4.6 million Macs during the September quarter -- both of which would represent a quarterly best.

Meanwhile, Munster noted that sales of iPods during the month of July fell just 15%, which was better than the 24% he had been modeling. He said the figures suggest Apple's quarterly iPod sales will fall to just 7.2 to 7.7 million for the quarter.