Needham-based Extreme Reach, a maker of software for transmitting video advertising, said Monday it has reached an agreement to acquire the TV business of Digital Generation Inc. (Nasdaq: DGIT) for $485 million in cash. The planned acquisition will include the advertising distribution business unit of Texas-based Digital Generation, a chief competitor to Extreme Reach. The deal follows a $50 million fundraise by Extreme Reach in May, which the firm had said would be targeted for making acquisitions. The…