The "bit" thing is how much very high speed memory the vid card has built in. This memeory is used to hold the texture details of the screen. In high resolution game settings you need lots of high speed memory. You also need lots of high speed memory for games where the scene changes very fast (like first person shooter).
The more memory the better in general. If you run out of high speed memory the video card needs to pull info from your slower system memory and the display may freeze or get jerky.
Also you need to look at how much memory (bigger is better 256KB, 512kB, 1MB since it allows more texture info to be stored) there is, how "wide" (bigger numbers are better 128 bits is half as fast as 256 bits wide, etc since they make for faster access) the interface is, and how fast (bigger numbers=faster clock and less time to get at the texture data) the memory clock speed is. (DDR3 is faster than DDR2)
Dog board 128kB of 64 bit wide GDDR2 or DDR2 800mhz.
Dream board 1MB of 256 bit wide GDDR5 1200mhz.
The clock speed gets hard to read becuase there are many difefrent versions (GDDR2 through GDDR5) and they get labelled differently becasue of the way overall speed is computed.
All of this is important if you want to run highest resoultion and max out the texture settings fo eye popping effects. If you're not that demanding you can throttle back and live with slower/less expensive boards.
Never appologize!
Everybody was a noob at one time.