For Tom's hardware, there are many articles explaining the elements of the system, and they put together some good comparisons of CPUs and graphics cards that show the best value at various price points:
http://www.tomshardware.com/reviews/gaming-cpu-overclock,2772.html
http://www.tomshardware.com/reviews/gaming-graphics-radeon-hd-6870-radeon-hd-6850,2782.html
They also have a suggested system article, though I find that to be a bit more opinion and therefore a rough guideline:
http://www.tomshardware.com/system-configuration-recommendation-51.html
Generally find some suggestions by others and do some investigation of the parts they suggest. You will be more satisfied with your final system if you have a decent understanding of the parts you put into it.
The forums on tech sites like overclock.net mentioned above often have many debates over system components, so if you can get some actual model numbers you can find some good information on the pros and cons of individual parts.
Just google the model numbers and there are quite a few enthusiast sites around that will discuss them.
Make sure you don't skimp on the power supply either, and that it has enough juice for the video cards you want. It is a part that is often given the least consideration, but can have a definite effect on the end performance of the system.