If thats all you do then you dont need much at all.
* Motherboard - Name brand - asus, gigabyte, msi are the three I choose between usually. I use an Asus M4A785TD-V EVO in my HTPC (about the same system I'm spec-ing here) and it even handles many games fine. Web games like facebook are a breeze. This even plays Wow on reduced settings. Notice this board has dedicated video memory.
* CPU - recommending an athlonII x3 445. I dont think you even need this much power but its at a great price point IMO.
* RAM - 3gb dual channel 1333 ( 4 if you want but then you should use 64bit os) The DDR and ECC must match what the motherboard needs. Ex: The Asus board I use uses DDR3 up to 1800 but 1800 and 1600 is OC (overclocked) and 1333 is stock speed. I got 1600 speed non-ecc unbuffered ddr3 (EEC is mainly for servers)
This is the kit I purchased: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
* Hard Drive - dual 1samsung spinpoint F3 drives. 1 as main, 1 as backup. (these are on sale at newegg right now for ~$58) Dual to keep all that music stored safely.
* CD / DVD - any 5egg SATA dvd burner
* Video - onboard, you didnt say you game so unless you do a dedicated card is not required.
* Power Supply - 350w should be more than enough. Get quality to protect your investment. Look for 5eggs, continuous power, active pfc, 80 certified, over and under voltage or current protection are things to look for. The antec 380D gets excellent reviews and is very reasonably priced.
* Network Card - onboard
* Sound Card - onboard unless you are an audiophile. I'll let other audiophiles chime in here
* OS - Windows 7 Home Premium 32 bit unless you want more than 3gb of ram, Then get the 64 bit. If you want to do a backup across your network then you need Professional version instead.
* Surge protection - don't forget this. I use a cyberpower 1080. Joules is the keyword here thou features line phone, network, and cable line protection may be of value to you also.