£600 to upgrade

Need some advice:

This is what I want, an Athlon 64 processor, 256mb graphics card, a motherboard, 1Gb RAM (currently have 512Mb DDR RAM), new case and possibliy a new soundcard but within this budget I may have to keep my Audigy 2 card I currently have and upgrade later. My hard drives are fine (I have 480Gb in total spread over 3 HDD's), got a new TFT so thats not a problem, CDR and disk drives are fine, ethernet & wireless cards are all good. I suppose I may need to buy 1 Gb DDR RAM so I'll end up having 1.5Gb in total unless you guys recommend just buying 512Mb and putting them together?

The PC will be used for a variety of things: Rendering architectural drawings, downloading music and eventually music production hence I may buy a better soundcard later on. So it needs to be an all round performer but I want it to render as quickly as possible.

Is there anything I've missed out? My budget is a bit flexible and I can add bits on later but ideally I want to keep it under £600 if possible.

Are there any barebones systems out there that are really good value at high spec or do you pay a premium for them to build it for you.

Where are the cheapest places to buy seperate parts from? Any comparison sites/shopbots you'd recommend for PC bits?

Thanks in advance for your help )