What are the technical specifications in buying a computer?

First you need to know what is purpose of buying system.

if it is for graphics design then heavy graphics card and processor are required same is for gaming with bundle of ram say 4 GB minimum RAM and atleast DUAL CORE PROCESSOR with atleast 1 GB graphics CArd.

but for simple documentation and web u simply need a processor with 2 cores and 4 gb ram , graphics card in this case is optional.

what i recommend is :
Processor: core i5 2300k
Ram: 4 gb ram corsair
Hard Drive: 500 gb hard
Graphics: 1 gb 550ti Nvidia graphics