Well the main causes of computer noise are:
CPU fan
Power supply
Hard drive
Case fans
Case fans
Dell makes their computer so quiet by using passive cooling where they can and by using ducting huuuuge usually passive heatsinks to minimize the amount of case fans...
There are a couple things you can do to make your computer quiter:
Get a ax-7, sk6, or gc68 heatsink and couple it with a panaflo
Mod your power supply and replace the fans and undervolt them to make them quiter
cut out any stamped case fan grills to decrease air turbulence
get a quiter hard drive like a barracuda IV(almost silent) or get a hard drive anti-vibration cage to decrease case resonance
Get a higher quality vidcard hsf combo and undervolt it
etc, etc, etc
Just some ideas. How far you can go with getting your computer quiet really depends on your system specs and how willing you are to sacrifice performance for quiet.
Myself, i got my computer (XP 1700, Geforce 2 GTS, 768MB RAM) down so quiet, the noise from my computer was being drowned out by my flourescent lamp and monitors...
Of course the price i paid was a relatively high cpu idle temp of 57C. Within spec, but a little toasty for my tastes. Im still working on a better compromise. The quest continues!