i am on pc: Gigabyte 965p-ds4, Core2Duo E6600, 2x1Gb 800 Hz RAM DualChannel, 2x320Gb Seagate HDD, Seasonic S12 500W PSU, Scythe Ninja CPU cooler + couple of Arctic cooling fans.
As for the 4 Gb RAM, you will have a problem on WinXP 32-bit as it cannt handle more than 3Gb of RAM... It is ok on 64bit WinXP..
If you will go for 3Gb RAM,. I would suggest 2x1Gb Dual channel and 2x512Mb Dual channel rather than 2x1Gb Dual and 1x1Gb single.
As for the CPU, personally Iwould suggest you to go for Quad-core instead of dual core.. The prices dropped rapidly, you can buy decent Quad core for a price of dualcore, now.
Dont spare on CPU cooler and other cooling stuff as if you want to have silent pc, you need to have good cooling system in there. Bear this in mind when you will look for fans, PSU and PC case. As for the PCcase, take the one with good airflow becasue your fans can work on lower RPM, then so it will be quiter than closed PCcase with fans on higher RPM...
Hope this helps.