Stick with NiMH... to run Lipo at the very least you'll need a new speed control, charger and of course batteries... figure at least another $200, not worth it IMHO at this point... pick up one maybe 2 more NiMH packs and just FLY

if your dead set on going Lipo spend some time in the Electric and battery forums... lots of GREAT info to be had.
John