guver nailed it with the first reply. However you can find pretty high capacity NiMH cells (4500mAh) for a good price:

http://www.all-battery.com/index.asp...S&Category=182

I bash with their 3800mAh pack and the cells are actually in the 1.2 ~ 1.45v range. Fresh off the charger the 7.2v actually packs almost 9volt! Just don't charge it with high amp or the battery would be too hot and would need a cool down first.

There are other things you can do to lower a packs resistance and offer more punch:

high quality 12 gauge wire and connector for the battery
silver battery bars and a good soldering skill

But better yet, 7.4 volt 4800 mAh lipo with a balance charger...