ALSA has a quite good support for USB audio devices, the only problem could be when you mix USB 1.1 audio devices with USB 2.0 hubs.
The CONFIG_USB_EHCI_SPLIT_ISO option in the kernel must be enabled to cope with that. Kernel version 2.6.25 or later does not have (or need) this option.
Since the NSLU2 has not enough power to play MP3s with a floating-point based decoder, I suggest using an integer-based one, like madplay.
If you want to remote control it, then you may want to try MusicPD.
Average CPU load when playing should be around 10%.
See also SlugAsAudioPlayer.