Here are some good pages to browse of stuff:
www.m-audio.com
www.motu.com
www.sweetwater.com (Lots of advice on their page if you hunt)
www.alesis.com
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The audio on mac computers is very good, actually. The OS X kernel is built with a low-latency audio driver (core-audio) which works very well. The newer macs' built in audio can do 24bit/96kHz. If you don't need to hook up to anything else (guitar, mics, etc.) then the built in sound is very good.
If you are going to get an I/O box, some things to think about are:
- How many inputs/outputs you need?
- How many XLR inputs needed?
- Will it work with your software? (e.g. ProTools hardware doesn't work so well with Logic, and Logic Express)
- Does it have balanced inputs?
And I'd recommend Firewire. USB transmits data in bursts, whereas Firewire is a constant stream, and much better for audio. Especially with multiple tracks at a high sample rate.