Pak,
1.) Try Newegg.com or gameve.com for your processor, they usually have great prices and if something does go wrong they are very responsive with their RMA processes.2.) I would not use eBay for a used processor- that is just a personal preference. Others may feel very comfortable with it.
3.) I use the Zalman CNPS7000B CPU cooling fan. They are very nice and quiet, but a bit of hassle to install.4.) 512 RAM should suffice unless you are planning on gaming., however for regular Internet use and watching videos, listening to music etc.. that should not be a problem.
Cheers, Harris C