Look over this website
Quiet PC
I think the biggest difference made to my PC noise was getting the 'SilentDrive Acoustic Enclosure - £24.00' You put your hard disk in it and it really does cut the noise down. The new 10,000 RPM drives wont like it though, due to heat.
Also the 'AcoustiCase - £89' will make a big difference.
For CPU get a 'Heatlane Zen Fanless CPU Cooler - £49 DO NOT buy Zalman, they suck! My brother had a 'silent' one and it was quite hearable in my studio!
and for the power supply keep what you have but open it up and change the fan for a 'Ultra-Quiet 80mm Fan - £9'
Put it all together and you have a £171 bill and a truly silent computer!!
If you want to save money scrap the PC case and just stick a lot of thick carpet everywhere you can (other than one hole for fresh air to come in, I leave my unused PCI slots open) inside your current case.
I use a very similar setup in my studio. The only difference is I soundproofed the case myself with 'Brown Bread Sound Dead' and some thick carpet and my CPU cooler is a Cooler Master silent heatpipe. If you use the stuff above from Quiet PC it will be even quieter!
For soundcards I use an Edirol DA2496 which is about £380, but you can get a cheaper card as you only want two channel output. Do look at the Edirol range though.