DesignTechnica has reviewed the Request Multimedia Nitro Pro Music Server.
The Nitro Pro is a 19 inch rack mountable high end hard drive based audio player. It has a Pentium III processor and 128mb RAM, a 40gb or 60gb hard drive, and a CD-ROM drive.
The downside to the Nitro Pro is that it costs $2500 with 40gb drive or $3000 with 60gb drive, for that kind of money you can buy a good off-the-shelf Media PC that can do a whole lot more.
My recommendation would be something similar to the discontinued Gateway 901 Media Center PC (available remanufactured for $1399) with 3Ghz Pentium 4, 512mb RAM, 250GB hard drive, DVD-RAM/DVD-R/CDRW drive, 128mb video card, media card reader, tv tuner etc.