ExtremeTech wrote a howto on building a nettop for under $500, you can check it out over here.
So why build a nettop? It's a much cheaper alternative to a full-scale desktop PC, particularly if you're looking for a system primarily to surf the Web. Of the components we used, the CPU and motherboard alone will only cost you $80 combined. And the new Intel Atom D510 processor (the first Pine Trail Atom processor) has the graphics (GPU) built right into the 1.66-GHz chip. The dual-core D510 also has Hyper-Threading, so it can theoretically handle up to four data streams at once..