Video Memory consists of the frame buffer and texture memory. It wasn't long ago when video memory was just 2MB, but with the abundance of cheap RAM out there today's cards have 16MB or 32MB of memory.
Types of video memory: SDRAM and SGRAM
Current cutting edge 3D accelerators use SDRAM or SGRAM. Synchronous DRAM is the generic clock synchronized RAM used in just about everything. The clock synchronization allows for sustained data access bursts without any extra synchronization needs. SDRAM is dual ported. Synchronous Graphics RAM is clock synchronized RAM made especially for video memory. It has a write-per-bit function that can write only selected bits in a word, and it also has a block write operation that allows it to write in several locations in a single cycle. SGRAM is only single-ported, but it can open two memory pages at once, which sort of simulates the dual port nature of SDRAM.
So, which one's better? Frankly, it doesn't really matter any more. Clock speeds are the most important performance factor, and both types are available in the same speeds. Almost all of the current generation 3D accelerators are using high speed SDRAM.
The frame buffer is part of the video card memory that stores the actual frame, the screen image that goes out the monitor. The size of the frame buffer determines what resolutions and color depths your video card supports. A 1600 x 1200 16-bit color resolution needs 1600 x 1200 x 16 = 30,720,000 bits, or about 3.66MB of frame buffer memory. Up the color to 32-bit, and the size of the frame doubles to about 7.32MB.
As you can see, that's not too much information to shuttle around at all, especially for a fairly high-bandwidth bus. So what's the fuss about 16MB, 32MB, and soon, 64MB of local memory on 3d accelerators?