Software does matter as well
If I were you I would also pay a bit of attention to the software you choose. I would pick up a program that has some processor saving features. For example Logic Audio Platinum have come out with version 6 (will be available this month) and it has a ton of special processor saving features.
It has track freeze. What this means, is that you can use the effects plug ins on track one (for example) and just load that track down with whatever you want... compressor, EQ, reverb, etc. Get it sounding just the way you want it... and then you can "freeze the track." meaning that Logic will create a fully rendored version of the origianl recording with all the effects on it and can play it back as one audio file. Otherwise the processor has to be adding the effects in real time to each track... this is really processor hungry. Then you can move on to track two and EQ it, compress it, do whatever, then freeze it. And so on and so on.
With the processor you have 350mHz, you will definitely get your 4 tracks (although i am sure you will want more soon) but as soon as you add a reverb effect and an EQ to the first two tracks your processor will choke down. This way you can extend the functionality and with a lot of creative techniques and a hell of a lot of tedious time spent freezing, listening, unfreezing, correcting and tweeking, then freezing again, listening blah blah... it will take a while to get used to, but in theory you could work out some good mixes with very little CPU resources.
Also Logic audio has the new feature of Off-line bounce down. That means that when you bounce down the final mix after you have everything right and you think the song is finished and you want to burn to CD to have it mastered (or just release it as it is) then Logic will bounce down all the trakcs with all the plug in effects and automation passes faster than real time. (meaning you don't have to sit through your whole song playing through). You can just click on bounce down and it will mathematically add up the tracks and put the plug in on. This way you can push your computer to the absolute limit of CPU power and the WAY WAY WAY beyond what it could normally handle and then during the downmix it will not choke down, because it is not in real time.
I hope you understand what I am talking about.
Do you already have a software program in mind?