You can still buy Nixx battery packs. The NiCd packs tend to be a bit expensive but they are available. I would stay away from the new Hydrimax NiMh packs as there have bee reported problems with the quality.
If you want to upgrade you charging equipment then you can look at Lixx batteries..You don't have to spend hundreds of dollars on a charger but you would want on with a display so you can read the charge/health info on the packs. What they call a "4-button charger" would work well.
You can use either LiPo or LiFe packs. As stated a LiPo will require the use of a regulator for most servos and receivers. LiFe or authentic A123 packs usually don't need a regulator but you would have to check the servos and receivers you are using to make sure they can handle the extra voltage. I use both LiPo and LiFe&A123 and for the ones that are difficult to remove I charge them in the plane with no problems. You really need to know how your charger works and how to properly charge and maintain these packs otherwise you will run into problems . That is how some catch fire is because folks are trying to charge a damaged battery or have the charger set incorrectly ad over charge them. Until you are comfortable with and know how to check the health of the packs you should remove them and charge them in a safe environment and monitor them through the charge cycle.