Great question. The amount of MB you use for this will be the amount of images your browser saves off the internet to your hard drive for faster browsing (it takes the images straight off your hard drive instead of redownloading them everytime). The only reason I would put the amount of MB to what you want is if you were runnning out of space on your hard drive. The bigger the amount of MB, the more images are cached and the faster your browsing will be for more sites. If you put that number very low, your broswer will save the amount of images space you set aside and then have to redownload the rest everytime. If you don't have any disk space problems, 100 MB is fine. Mine is currently set to 80 MB.