24 bit 96 or 192k sample rate cards are all you need. I use a $19 digital I/O card in one PC, works great, 24 bit accurate.
Modern converters like Burrbrown and ESS can only do about 21 bits A/D, maybe a a couple more for a DAC. The rest is noise. BurrBrown also makes 32 bit converters, but the specs don't reach the resolution of their best 24 bit parts, it's all marketing BS.
Digital processing does work better at higher bit depths, but that isn't the audio bit depth, just the processors.