I would suggest Lipos. They handle load way better. It takes some soldering, but a Lipo will last longer, is much lighter, and can handle high discharge bursts. The drawback is you can't completely drain them and they take a special charger. I use a voltage alarm on mine. A 1S Lipo is 3.6V and they come in 900mAh. Hobbyking.com is a good online store for them.
I tried several motors. The best replacement is PN Racing 70 turn Mini-Z RC motors. Get rid of the Trustfires. They cannot handle the wattage demand from good motors spitting a dart out. Switch to a 2S 7.4V Lipo battery. They take a special charger you can get online for about 30. 1800 Mah is a good fit. I use Turnigy Lipos from Hobbyking.com.