all depends on the software you use. For instance, Nuendo would run no problem, but ProTool would crash at first plug in. Sonar likes 1gb, but runs with 512.
In my beginning audio courses in school, I used to use a 500mhz with 512 ram which ran 24 track playback and record in Nuendo without a glitch. They eventually upgraded it, but it worked great.
I suggest at least 1.5ghz and 1gb ram. But, again, that depends on software and the plugins you use.