Ley me see if I can picture this:
You want to compose tracks using both soft synths and hardware.
Softsynths: Basically, this is the way i look at. If I want to use more active vst's in a track, then CPU begins to play a big role. On My DELL 9200 (1.6 pentium m, 1gb ram, 5400rpm drive), I was able to use 2 instances of Reaktor, 2 instances of Korg Legacy, 3 instances of SUperwav 8, 3 instances of battery, each with two samples, 12 misc. effects ( 2 of which were reverb) and one mastering effect, before having to start bouncing to audio.
My recommendations are as follows:
1. Get as fast a processor as you can.
2. Get as much ram (at least a gig)
3. Get the fastest hard drive possible. (7200......you're safe)
Now, if you are doing low track count songs, meaning less than, say less than 8 tracks, then you really don't need an external hd. We use the HD when we do live performances, but then again, it depends on how intensive you song is.
I have a few musician friends who have two or three external hds They do record direct to them and they are for storing their audio files and sample collections.
The thing is, recording, whether it be live instruments or soft synths are two different things, but both require a powerful processor, memory, etc...if you don't want laggy, glitchy performance.
WIth Softsynths, most programs will allow you to bounce to an audio file, whether it may be a single track in the program or the entire song, so eventually, you will be dealing with audio files. I you have, say, 100 audio files that you are mixing, having them stream from a fast external drive is a good thing.
edit: I went back and read the previous posts in more detail
Okay, if you are working with a software synth, eq. Superwave8, which is a synth (no audio files , it is not a sampler, the sound you hear, unless you bounce to audio, is the synth. Cpu and ram are the key factor here. If you are using a sampler, cpu and ram are still the main factors, but now it depends on the sampler. Some samplers stream the audio, so if you are working with big samples, then having a faster drive will be better. Having firewire will be better.
But all these things are not really necessities, becasue ot all depends on what you are doing. If you are recording two tracks simultaneously, then doing over dubs, one track at a time, you don't really need the 7200hd (5400 is fine) and you don't need an external. You might want to get the best interface you can get, and more ram.
If you want to get an external drive to store stuff, that's a good thing too.
In my home studio, I don't use any external drive. I store everything in my internal drive. I would say most of my songs have on average 10 audio tracks. With 5 to 10 smaller fill tracks so that's 20 tracks on some songs. These are the mix down files just before mastering. I've never had any problems (knock on wood) since I had the Dell.