I Hate Computers

Okay.
Here's a basic rundown of what you want for a modern computer for writing music, and doing a bit of recording in the level of 'enthusiastic amateur'. I assume you don't want something uber pro, since you seem to be planning to leave the actual mixing of whatever it is you write up to me, so I'll try and compromise.
I've omitted prices - they will vary wildly compared to local ones due to the fact that Australia sits geographically right next to all the countries that actually mass-produce computer parts (esp. Taiwan), but further away from where the licensing/distribution type stuff happens.

CPU: AMD Athlon XP 2700+ (and suitable motherboard in the area of £80 - no frills, but not dodgy)
Athlons are not quite as powerful as Pentiums when it comes to certain instruction-specific, math-intensive programs such as 3D rendering, information processing, etc. But they represent considerably better value for money for straightforward, 'universal' tasks - like doing reverb on something in Cooledit. The 2700+ is currently at the kink in the price/speed curve - any faster and it starts getting inordinately expensive.

RAM: 512MB PC2700 (that's 333MHz @ DDR).
That should cover ya. If you're planning on maybe using lotsa soft synths or whatever (unlikely I guess), you might want to consider a gigabyte, but it should be OK. Brand not really an issue; if the memory doesn't work, it doesn't work - you can take the bugger back. Big brands like Kingston are a LOT more expensive than no name memory.

Videocard: Unless you wanna play 3D games, it's all good. This matters not. Hell, my card's a low end TNT2; they were crap when I got my computer, and that was nearly four years ago - I still feel no need to upgrade.

Soundcard: Well. Wanna use the good old soundfonts and so forth? You'll have to get a Soundblaster Audigy series card... (probably a 2). Very similar to the SBLive, but with no sample memory limit (JIVE for soundfonts), and just better recording quality. Better signal to noise ratio. If you want to be the ubercool, you could get one of the platinum models, which has either an internal or external unit with a bunch of inputs and outputs on the front. This will come in handy for recording from mics, the v-amp, MIDI keyboards, so on and so forth.
There's heaps better shit out there than the Audigy, in terms of raw audio power and fidelity, but it gets big money styles. I don't think you need it. Plus you don't get the synth.. at all. Even with the Audigy, megafont may not work (haven't tested that yet), so you may have to buy some kind of external synth anyway. (blah)

Monitor: Well, this is obviously down to personal preference. CRTs these days are so ridiculously, incredibly cheap, it will barely affect the overall price tag. LCDs on the other hand are still quite expensive. I personally don't like them because you can only run them in one resolution or it looks shit. CRTs ARE being phased out, and in fact the smallest size you can get is 17" nowadays.. you can get great ones for teh uberbargain.

ROM drive: If you're going to be copying CDs, I'd point in the way of a DVD drive plus a burner; or else you could just get a combo drive which burns CDs and also reads DVDs. I think the latter option is generally cheaper but prices are odd things. DVD burner = not really something you need, is it?

Hard drive: Bleh.. Computers these days come with 160GB drives. I've got a total of 55GB and haven't run into the limit yet; it depends on what you want to do with it.. Audio recording is NOT very space intensive in this day and age. Hard drives are pretty dang cheap though so I'd just say go for an 80GB one, 7200RPM obviously with an 8MB cache. NOT Seagate brand; they suxors. Beyond that just find the cheapest. They should only differ by a scant few pounds.

As for sounds... if you get an Audigy 2 after I disover that megafont works with it; I guess I'll have to send over the pile of soundfonts... I haven't downloaded any more in ages, but the ones I've got seem to be good enough because they're just backing tracks for 'real' instruments anyway and don't need to be anything phenomenal.. just not shit.

If soundfonts end up being the way of the past; you will have to get yourself some kind of proper synthesizer. Either an all-in-one keyboard type deal, with a little screen and a shiznitload of buttons, or a stand-alone synth module, which is like the aforementioned but without the keyboard part. Basically a box that you send MIDI commands to, and it sends out sound... like the synth part of the SBLive but in a box. You should be able to get yourself a semi decent secondhand synth for a good price.. electronica is BIG in England (as you've no doubt discovered) so synths are everywhere. BUT.. Be prepared to be not blown away. They are often geared towards technoey sounds that are designed to be fucked with; and the 'real' instruments are more often than not second rate (i.e. not as good as all the soundfonts I have). The advantage is you have a standalone unit that won't fuck up and complain about MIDI ports and whatnot, etc.

Foobar.

I hate computers. Lots.
I was addicted to them for a good... 12 years. They're too complicated, bogged down in commerce and multiple standards. And they FUCK UP. Of course, we can't live without them. But I think you should try to make computers as little a part of the musical process as you can. That's why I'm learning guitar. I'll still use the computer to come up with ridiculous O-chords, since they are impossible on guitar, but whatever can be transferred away, shall be. Hence the whole wanting to start a band thing. Computers are stale, life-sucking evil things. Gah.