The propulsion for the Pluto mission has alread been selected and as you suggest it's going to be conventional with a huge kick from an Atlas rocket and a Saturn (Jupiter?) gravity assist. The power supply will be radioisotope thermoelectric generators which rely on nuclear decay. I think it's slated or launch early next year.
The launch window opens on January 11, 2006. Jupiter Gravity Assist follows in 2007. Pluto-Charon arrival is slated for 2015. Without the RTG, the mission is hosed -- you can't go solar past Jupiter. However, the RTG will keep the spacecraft at a nice toasty 10 Celsius even at Pluto.