What the specs mean
Processor clock speed: Much as with PCs, a higher clock speed for the graphics processor does not automatically mean it runs faster. A 400MHz processor that can do twice as much per clock cycle as a 600MHz processor will outperform it. In terms of rendering speed, the per-clock performance of the graphics processor is indicated by the number of pixels it can process per cycle.
Memory size: This is the amount of memory on the graphics card, to be used exclusively for graphics operations. Nearly all chips support multiple memory configurations, some up to 512MB. The effect of memory size on performance will vary depending on the application. The graphics processor can talk to the graphics card's memory much quicker than it can talk to the PC's memory, so the more memory on board, the less it has to go to main memory for the information it needs to render a scene.
Generally speaking though, if you currently have a 128MB GeForce 6600-based card, for example, and want to upgrade to a 256MB GeForce 6600-based card, you may not notice a performance difference at all during gameplay unless you are playing games with big enough texture sizes that can take advantage of the extra memory, such as Half-Life 2, Doom 3 and Far Cry.
Memory bandwidth: This is the speed at which the graphics processor can talk to the memory on the card. One of the big bottlenecks in 3D performance is the speed at which the PC can deliver information to the graphics processor. Faster memory means that this bottleneck is eliminated, resulting in faster rendering speeds. There are two types of memory used on current graphics cards - DDR and GDDR3. GDDR3 is faster, and better able to keep the graphics processor occupied.
Performance will also be affected by the width of the memory bus. A graphics card with a 128-bit memory bus can haul double the amount of data between the memory chips and the graphics chip than a card with a 64-bit bus. Generally, only mainstream cards have a 64-bit memory bus.
Shader model: DirectX Shader Models give developers a great deal of control over the appearance of the scene on screen, and can be used for various neat effects like complex shadows, reflection, fog and the like. Microsoft has improved the Shader Models over time, giving more power to the developers (and thus greater potential to make scenes look realistic). Shader Model 3.0 is the latest version, found in DirectX 9.0c. At this stage, only Nvidia supports Shader Model 3.0.