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Recording with 18, 20, 22, or 24 bits will always take 50 percent more hard disk space then 16-bit recording, and will also greatly reduce the maximum number of audio channels that you will manage with a given PC. The increase in audio quality will depend greatly on the individual system, its converters, and the quality of the source material.
Moving up to 20- or 24-bit PC recording is only sensible if your sources will benefit from it, or if your customers demand it. If you intend to do a lot of hard disk audio recording of live music, you will certainly find it easier to work with more headroom available, and with 24-bit converters you could probably work with 20dB or more of headroom without compromising the signal quality too much. Of course a wider dynamic range isn't the only advantage of 24-bit -- the overall quality of the converters may well be better too.
However, much modern music is heavily limited or compressed, and there seems little point in using such converters with this material. In fact, some people claim that 16-bit recording at 96kHz can provide better results than 24-bit at 44.1kHz. This is likely to be because of the very sharp low-pass filter cutoff needed at about 20kHz to avoid artifacts when recording at 44.1kHz. If the material is recorded at a sampling rate of 96kHz, the roll-off need only be comparatively gentle, and even when converted down to 44.1kHz afterwards, the top end may sound more natural. For this reason, an 88.2kHz sampling rate may be useful, since it is likely to be easier to convert later to 44.1kHz than a 96kHz sample rate.
Ultimately you have to use your ears, and not rely on figures alone. In fact many people would put this the other way round -- you should use your ears first, and then confirm the figures afterwards.
Once you move beyond the theory into the real world of actually recording analogue sources, other factors enter the equation as well, and if you are contemplating moving up to 20- or 24-bit recording, it is worth first considering the rest of the recording chain. If you have a cupboard full of expensive low-noise mics and ultra low-noise mic preamps in a soundproofed studio with high-quality wiring, you are likely to get wonderful results when recording at 24-bit resolution. The reality, for many people, is rather different. Developers have found it difficult to test 24-bit recording quality with their latest soundcards, simply because few rooms are sufficiently quiet enough to give a background noise level that is low enough to record acoustic signals with a dynamic range beyond 16-bit levels.
Low-noise mics are generally quiet enough to benefit from being recorded at 24-bit resolution, and it's easy to assume that digital synthesizers will do so as well. In many cases, however, the improvement over 16-bit will be negligible. The source samples used in these instruments are generally 16-bit samples, recorded at 44.1 or 48kHz; they may, however, be processed within the synth and its onboard digital effects at a higher bit depth to avoid rounding errors in calculation. If they are then fed to a 20- or 24-bit digital output, it may be worth recording digitally at this depth. However, most digital synths still lack digital outputs, and seem to use D-A converters of no greater than 16-bit resolution in generating their analogue output -- so there is little point recording their analogue signal at anything other than 16 bits, as its dynamic range will not exceed what can be captured by 16-bit recording. In addition, the background noise of many synths may also be annoyingly high in some cases.Perhaps it's not so surprising that few synth manufacturers publish technical figures for noise. In any case, the moral would seem to be to check up, if possible, on the dynamic range output by your digital synthesizer before choosing to record it at 24-bit resolution.