Cowtown is 20th-largest city By Aman Batheja Star-Telegram Staff Writer Fort Worth is on the move on the population charts. So is Dallas. They're just going in opposite directions.Jumping past five cities -- including Boston, Seattle and Washington, D.C. -- Fort Worth is now the 20th largest city in the country, with 585,122 residents, according to 2003 Census figures released today.Dallas, meanwhile, was supplanted by San Antonio as the eighth largest city in the country. Dallas, which gained fewer than 2,500 people between July 2002 and July 2003, dropped to No. 9.Fort Worth has been enjoying a robust growth rate...