Digital Audio processing is a resource intensive task that relies heavily on the processing and I/O capabilities of a system. I would strongly recommend a Pentium class machine as a minimum.
If you are going to be encoding from an analogue audio source via the line or microphone input, a PCI soundcard will give the best results. The I/O performance difference between an ISA and PCI based card is significant, over 132 MBytes/sec for PCI (quote taken from the PCI-HOWTO). Naturally, the better the quality of the soundcard in terms of its signal-to-noise ratio, the better the encoded MP3. I've been using the Soundblaster PCI128 and just switched over to a Soundblaster Live Value; both cards give good audio performance, but the Live has significantly better S/N ratios, good enough for semi-pro audio work. Remember the old data processing maxim:- garbage in - garbage out!
Creative have a Beta driver for the Soundblaster Live! which can be downloaded from:
http://developer.soundblaster.com/linux/
When recording analogue audio to a hard disk, more commonly referred to as direct to disk or d2d recording, the performance of the disk, and its interface is critical. If you are using an IDE based based system, mode 4 or UDMA is preferable as the transfer rate is sufficiently high enough to provide reliable data transfer without problems.
The ideal solution would be to use a SCSI based system as the drives and interface have far better throughput capabilities, a sustained 5MB/sec for SCSI 1 through to 40-80MB/sec for ultra-ultra2/wide SCSI. IDE can peak at anything from 8.3 MB/s to 66 MB/s for Ultra DMA mode 4 but these speeds are peak, average transfer rates will be slower. If you can find, or afford, an AV SCSI drive, go for it. AV drives have had the read/write head system optimised for continuous data transference; other SCSI and IDE drives normally cannot sustain continuous data transfer as the write head heats up!
Naturally a drive that has cache will give more consistent results than one that doesn't, as the cache will act as a buffer if the heads do lift or it cannot handle the throughput.
If your drive isn't up to spec, your recording will suffer from dropouts and glitches, where the drive failed to record the signal. If you are recording one-off events, such as live performances invest in a good SCSI based disk system.
Another cause of d2d dropouts is a heavily loaded system. Background tasks can cause the system to momentarily glitch. Its recommended to run as few background services as you can, especially networked based services. For more information about setting network services, and startup scripts please refer to the SAG and NAG guides.
Virtual memory paging will also cause glitches, so run with as much physical RAM as you can, I'd recommend at least 32 Mb, but you may well need more.
For those wanting to extract the most out of their system, optimising the kernel probably wouldn't do any harm either.
For streaming MP3's the better the network card the better the throughput, naturally a 100Mb interface will give better throughput than a 10Mb interface :)
While the hardware specifications above will give you a decent system to encode audio data, don't discount using older, lower spec kit if that's all you have access to.
It'll be a good challenge for a sys-admin to tweak a low-spec system to give good results, and the end result will probably be a happier Linux box.
Another important issue is the audio cabling. Cheap, poor quality cables and connectors will result in poor recording quality. If your soundcard has the option to use phono, sometimes referred to as RCA connectors, use them. Gold plated contacts will also help maintain audio quality, as will keeping audio cables away from data cables as there will be a chance of interference between them.
But don't forget, spending a fortune on the best audio cabling will be lost if the rest of the system hasn't been optimised.
For encoding MP3's from CD-ROM, the speed or type of drive will determine the time taken to read the raw information from it. A single speed drive will probably be too slow for all but the most patient.
Your CD-ROM must be connected to your soundcard if you want to hear what you are recording, either using the internal connector or by connecting headphone's to the headphone output, although you will not be able to listen to MP3's through the CD-ROM headphone socket!
For detailed instructions on setting up soundcards, now would be an excellent time to read the Sound-HOWTO.