if your just recording it as is you should be fine with the cheapest thing you can get. even if you are using some effects you should be fine. I would say worry more about the soundcard to get the input accurate. the cpu isn't going to make much difference for any of this. it's not like video encoding.
most onboard soundcards will use cpu functions but many decent add on soundcards do the work for you. either way audio isn't that hard on the cpu. if your curious about this find a program to record some audio and apply all the filters you want and all the effects. try to stess it out as much as you can and watch your cpu usage. chances are you won't even get above 10-15%.
as I was saying I used a p3 300mhz compaq computer with an onboard soundcard hooked up to a mic for a cheap accoustic guitar that I had way back in the day and it worked just fine.
I'm recording and editing music for bands myself, and the CPU isn't all that important. RAM will be the most important part of the recording. It will take up very much memory especially if you are using multiple tracks with digital gates, limiters, compressors, equalizers and all the other (in my eyes) necessities.
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