basicly it is giving the option to store the frequently used data in memory (RAM cache) or on the hard drive (Disk Cache).
You probably won't notice the difference as to browsing speed/functionality based upon RAM cache vs Disk cache.
RAM cache will tend to be quicker but uses the available memory in your computer. If you have a large amount of system memory (RAM) and the fastest possible web page browsing is a high priority for you (moreso than other applications running fast), go ahead and up it to a larger number (I wouldn't go beyond about 1/10th of your total system memory though).
Either way, I would recommend upping your disk cache to around 256MB. Your's seems rediculously low unless you don't spend much time in your web browser.