Forget building a future proof upgradable system, technology moves too fast. Buy a cheaper motherboard and processor and in a year or so do the same again, you will end up with a more up-to-date system than you would have had you gone for the more expensive option and your total outlay will probably be less.
Firstly ram, go for a 256 chunk at least. anything less is not worth the savings.
Hard drive, the bigger it is the more junk you will put on it, and the more suicidal you will feel if you ever lose all the data on it. I personally would not put bigger than 30 gig in a system at the moment.
Go for onboard sound and graphics and your system cost will be far less, you only need separate sound and graphics if you will use them, e.g. for gaming (but this will require a top notch system you are prepared to upgrade every time a new game comes out), soundcard may need upgrading if doing music work or a better graphics card if you want something special such as video/tv out or a TFT/plasma screen driver.
Forget the fastest CD burners, a 24 speed drive will write a CD in about 4 minutes, is it really worth paying the extra just to save a minute per CD ?
As for processors I recommend AMD (yes this is a rather shameless product placement !) I have an Athalon processor, before that a Duron, I would fully recommend them.
Please bear in mind though that these are only my personal thoughts. Try to have a look at someone else's system that is at what would be described as 'very basic entry level' by the shops that sell full systems before you make up your mind. Also bear in mind that these shops will try and make you think you need a system powerful enough to power NASA control.
