*Review: Extremetech (
http://www.extremetech.com/article2/...60427,00.asp)*
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One approach to this is to buy a relatively complete, barebones small PC. The champion of this approach is Shuttle, with their roughly cube-shaped PCs, such as the SN27P2 or the company's SD37P2.
Shuttle's tiny PCs are great unless you want to add more expansion cards or bigger graphics cards. The next step up, if you want more slots, is to move to a microATX motherboard and case.
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