Oops. I forgot to mention something. The ram you listed is Registered, ECC memory. It is used for servers. You want non-registered, unbuffered non-ECC memory. What you need really is the most common type. The crucial ram you listed actually has a 9th chip used for parity checking. It's useful for servers but actually runs slower than standard ram.
Try this from Crucial (Micron):
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=52C94B87A5CA7304
Socket 939 motherboards usually support up to 4GB of ram, using all 4 slots.
Check the specs on the motherboard you want to buy.
However, unless you will be using a 64bit operating system, you will only need 2GB. A 32 bit OS can only address 4GB max, and in practice will only acknowledge 3-3.5GB. In addition, if you open up Windows task manager and look at the performance tab, you will see your peak commit charge is less than 2GB. It's probably less than 1GB. Your total (the current usage) is likely to be 250-300MB or so, since you are surfing the web. As you can see, 2GB will be plenty.