This is a highly subjective issue, and so I will attempt to highlight the 'features' you should be assessing when selecting a video card, I will not demean the reader by trying to push any one brand, but provide unbiased, factual information about varieties of graphics cards as well as the features which will give the greatest performance.
First and foremost is the type of video card you buy, there are AGP cards and PCI-Express Cards, depending on the slot available on your motherboard. This is an easy one, PCI-Express cards have a higher potential bandwidth, so Pci-Express (PCI-E) is the bet choice here.
Next, is the type of memory your video card has, the potentials are DDR, DDR2, DDR3 ranked from lowest to highest speed (DDR3 is the quickest), however the memory bus will also affect your overall performance, so whichever memory type you end up buying try and get the maximum bus width, for example the options here are 64, 128 and 256 bit. 256 bit gives massive bandwidth advantage over the other two. Also memory speed in Mhz will give you an idea of the quality of Video memory you are buying. Higher is better.
Next on our list of selections is the core speed. This is fairly simple to ascertain as the numbers are given in plain Mhz, the higher the better. To make sure the most benefit is being made of this 'raw' speed, please try and buy a modern chipset (Currently 7900+ in Nvidia for example).
The final, and often most obscured part of purchasing a video card are the DirectX features (A modern one should support DirectX 10) and the number of Pixel Pipelines. This number alone will have a MASSIVE impact on performance, the more, the better. Current cards have up to 16 pipelines, older ones may have 1, 2 or 4. The difference is HUGE, as each individual pipeline does a specific task in rendering any image, so more pixel pipelines will give you much higher frame rates in any 3d application.
I hope these tips allow you to avoid manufacturer 'spin' and let you see the true value and performance potential of each Video Card.