THG of course does excellent reviews of video cards.
This site lists 13 good sites for video card reviews (one of which is THG):http://www.upgradevideocards.com/reviews.html
Also note that you can go to the THG montly article listing the best video cards for different budget levels, along with a brief description about the performance, AND a link to their review and benchmarks of the card - a very handy all in one reference and good place to start.http://www.tomshardware.com/reviews/gaming-performance-radeon-geforce,2997.html
You might also search the THG site further as for many popular models they have "shootouts" comparing the performance of different brands of the same model card. The perfomrance differences between cards of the same model is usually quite small, but you often find bigger differences in temprature because of different cooling systems, and often can see the performance advantage of overclocked cards compared to the reference design.