Two things, decide what your budget is and decide exactly what you need this pc to do. What are your priorities?
Web browsing, word processing, emailing etc are easily dealt with.
nvidia's graphics drivers are generally acknowledged to be more Linux- friendly than ATI's.
You mentioned games which is a bit disconcerting. Linux is not big on games generally speaking, so you would probably be best using Windows if gaming is a major priority.
With a dual core processor which supports virtualisation, you can run Windows in "Wine" on a Linux pc.
Here is another link which may be useful:
Alternatively, you may be able to buy a ready built pc (new or used) which is Linux-compatible and is favourably priced.