From experience, I would say 60 db of gain is adequate for most application for a cartridge rated at .25mV output. I run a .30 mV cartridge into a phonostage with 60 db of gain and there is absolutely no noise issue at all. I do have to crank the linestage fairly wide open (13 db of gain) to feed my Stax headphone amp which apparently has very low gain, but with speakers, gain is not an issue.
With a low output cartridge, whether noise becomes an issue can be dependent on HOW gain is achieved. With solid state, noise is usually not an issue (the one area where solid state is clearly superior). With tubes, getting such high gain using active gainstages can result in high noise level. That is why a lot of people use step up transformers in front of the first active gainstage (transformers do not add noise) in tube phonostages. My tube phonostage has such a step up transformer built into it.
