7.4v Lipo will get you about the same performance as the 9.6v NiMh, depending on the gun. 11.1v will give a significant performance boost, possibly beyond the guns capabilities, again depending on the make and model.
Fit the largest battery you can, with at least a 25A discharge rating. Unless you have to fit a specific airsoft shaped battery (eg crane stock or buffer tube) then look for batteries from RC suppliers rather than airsoft. They will generally be higher performance and better value. There is plenty of further information on the forum if you search for it. This includes the supporting mods you can do to get the most out of the lipo battery, eg low resistance wiring, connectors and mosfet.