I'm compiling a list of Congressional Districts in 2004 and by my reckoning (with 238 in so far) these are the ones most likely to change hands:
American Samoa 1
Missouri 3
California 20
Colorado 3
Georgia 12
Illinois 8
Louisiana 3
Indiana 9
Connecticut 4
Colorado 4
Minnesota 6
Connecticut 2
Indiana 8
Indiana 2
Louisiana 7
Pretty good. I'd add the 8th congressional district in Washington state, currently represented by congressman Dave Reichert -a Republican.
His district covers much of King County east of Lake Washington- and was won by John Kerry last year, and is becoming increasingly Democratic, especially on the national level.