Not all of us need the high speed graphic and fast processors that are needed for most of the best computer games out there today. When you surfer the net you don't really use that much of the processor speed that high end machines have, even when you are watch videos online.
This 11 page article, so get comfy, goes though the process of pick out all the components need, one at a time, to build a PC from scratch with new parts, for under £200 ($350), excluding the display and software.
I used to work in IT and spec'd compentent for video editing suites so heve really had to look at the low end range before, and that's low as in price, not spec, these PC will still keep ahead of 99% of PC user's.
I have already started hunting down compentent to rebuild an empty PC case that I currently use as a door stop.