I have successfully multi-tracked with cool edit pro 2.0 running on a 166 mhz computer with 128 mb of ram. Therefore, I must assume you will be perfectly fine with the setup you mention.
The things you will lose on a slower box are real-time effects processing that involve reverb or filters, or synth VST's, etc.
I am currently using a 600 mhz PC as my dedicated multi-tracker, and it has performed flawlessly these past 7 years, despite my growing awareness of longer render times in post processing.
Since I have it properly configured for good recording, as long as the motherboard lasts, it will continue to be my workhorse. Getting anything newer before then would just be frivolous spending.
So if you do decide to get that Mac, make sure it has a good studio quality soundcard, and you should be set for many years.