A: Visit Crucial to find out what kind of memory you need. They have an online configurator and a System Scanner you can download which tells you what kind of memory your system needs. The latter also shows which memory slots are in use and which are free. I highly recommend using this tool.
Crucial is the retail arm of Micron which offers high quality memory and comes with a lifetime warranty. Be sure to check out their specials. They frequently offer free shipping on orders over $40.
You can often find better deals at Newegg. I've had good luck with Corsair ValueSelect RAM which also comes with a lifetime warranty. I've had good luck ordering things from Newegg.
Other places to check memory prices:
Steve also has a selection of memory pulled from old systems. Due to volatile prices we are not able to stock newer memory.