If you have a buddy that is good with working with things electrical/electronic, you might check out replacing your current batteries with Nickle-Metal-Hydride batteries. They cost about twice as much per cell as NiCads but also can store about twice the amp-hours. The drills with NMH batteries are just starting to hit the stores and eventually they will replace NiCads and the price of NMH cells will come down.
Either NiCads or NMHs have 1.5 volts per cell so 4 cells equals 6 volts. The amp-hour rating (or actually milliamp) of a cell determines how long you can run. Recharging either battery takes the same recharger. NiCads are about $3 per cell and NMHs about $6. Personally, if I had a drill that was in otherwise good shape and just had bad batteries, I would just replace the batteries with NMHs. NiCads do develop a memory to where you cannot get all the power out of a cell and can otherwise only be recharged so many times without losing the ability to offer all their power. NMHs are not supposed to be as bad about developing a memory but they have not had nearly the amount of real-world experience.