The Dixie Chicks charmed when they joined James Taylor on stage and brought his usually dull ''Sweet Baby James'' to life with sweet countrified harmonies; they also worked wonders with their sassy version of Dylan's ''Mississippi.'' And frontwoman Natalie Maines came up with a priceless explanation for her refusal to take back her infamous comment about being ashamed that President Bush is from Texas: ''He'd just call me a flip-flopper, and I know he doesn't like flip-floppers.''