How much PC is enough?

That's a pretty nice chart to have. Looks a lot like the Excel spreadsheets I put together when doing builds (though I include pretty much everything you mention above). The one thing that it is missing for me is the ability to specify a tax rate. It would be very simple to add a box at the top next to the country selection that let you specify the percentage of tax for your given country/state/province. When I do a build I want to know what exactly will have to come out of my bank account. The only other thing I calculate into the cost of the part is shipping but that's overly complex, and beyond the scope of the guide's intent I think, as it means knowing who's shipping the part, where it's going, customs fees, etc.