1 gig is fine right now. I have one gig and it's not even 400 Mhz ram (266)

If you seriously want a silent computer, go water cooling.

I think you might find what you are looking for if you go to one of the non-major-name-brand stores. I can get a decent computer for about 700$ (cdn), and all you need to do (I suppose) is buy the parts for the upgrades (+300-500$) .

If a UK dollar is still 2x Cdn (which it is), you should be able to easily afford it. In fact since you already have a lot of the hardware, let's try again but using parts last I checked:

Case: 100$ if not less (£50)
Mobo: 200$ (£100)
Processor: 500-600$? (£250-300)
Video card (decent): 300$ (£150)
Ram: ??? (no idea what you want)
fans: cheap cheap cheap. Maybe £10 max each. I'm thinking £7.5

And the rest you aren't upgrading. Still, that's plenty of money available leftover. I think anyway, assuming prices are the same.