What's the maximum number of powered rails in a row that's useful?

Solution 1:

For inclines in 1.6 (tested), one powered rail at the bottom of a slope will boost a cart with a player in it up 6 blocks before needing another boost.

EDIT: TaviRider has made another great video showing all the powered rail things in 1.6.

Solution 2:

To boost the cart speed from zero to maximum, you need 4 powered rails in sequence.

To maintain maximum speed you need one powered rail each 26 blocks (25 blocks of rail, 1 block of powered rail).

To maintain maximum speed on slopes, you need to power each second block on the slope.

Solution 3:

All sorts of good related information: