The simplist thing to do is swap the 12/25 for a 12/27 but I don't think that will get you low enough.

As SK recommended, I also recommend replacing the RD with an XT or XTR, and putting on an 11/34 casette. (These are mountain parts, but they'll work just fine). I ride the hill country and I use a 50/34 in front with an 11/34 in back. This too is assuming you are running 9spd, but I know that terry is spec'ing their new Isis sport with a 10 spd 11 or 12/34 casetted by SRAM so it must also be possible for 10 spd.

If you do this I don't think you need to mess with your front chain rings at all.