Apple's new Mac operating system available Aug 28

* Snow Leopard release earlier than previously expected

* Mac shipments rose 4 pct in June quarter

SAN FRANCISCO Aug 24 (Reuters) - Apple Inc (AAPL.O) said on Monday its new operating system for Mac computers will go on sale this Friday, a bit earlier than expected, as the company looks to continue to expand its share of the PC market.

Apple's new Snow Leopard software will sell as an upgrade for $29, starting Aug. 28. Apple had previously said it would ship in September.

Apple sold 2.6 million Mac computers in the June quarter, up 4 percent from a year ago.

The company is the No. 4 PC maker in the U.S., according to research group Gartner, with 8.7 percent of that market. It does not rank in the top 5 in global shipments.

Apple rival Microsoft's (MSFT.O) new Windows 7 operating system is due in October.

Shares of Cupertino, California-based Apple fell 75 cents to $168.47 in afternoon trading on the Nasdaq. (Reporting by Gabriel Madway; editing by Carol Bishopric)