If you go to lith-ion, remember the volts are higher, I prob wouldnt run it higher than 2 of those..even then you may need it regulated down. If you want to use all the slots, talk to the people at Radioshack, they will give u the right diodes or regulaters u will need to knock ssome of the voltage down. If ur worried abt pooing the atty I prob wouldnt take it higher that 7v at max charge and I hear thats even pushing it.
Check out the atty mod forum, people will be coming up w/a new HV atty soon, I cant wait then we can really have fun modding!