If your intent is to buy cheap and upgrade later, then buying piece meal is not the cheapest way to go. Plus, you do not know if the pieces will work together or not.
This link will take you to eBay...There's a Build-To-Order page there.
Click on the AMD or the Intel barebones button. I saw a computer there for $169.00.
Click on the System Upgrade link and change what you like for the price you want to pay.
eBay: Intel Dual Core 3.4GHz 1GB DDR2 DVDRW Desktop Mini PC (item 220109390604 end time May-11-07 20:45:00 PDT)
Have Fun...