Get a 7.4 lipo and a lipo charger. It'll last longer than a NIMH or NICD battery.
http://eliteairsoftbatteries.com/produc ... -pack.aspx
Hobby king does have them a bit cheaper, but I like EAB. They're local to the Portland area and they ship very fast.
[shameless plug]
I also happen to be selling a lipo charger for cheap. Shoot me a PM if you're interested
[/shameless plug]