System/SDRAM Frequency allows you to run your RAM at a different speed to the FSB.
Say if your FSB is 133MHz, then for 1/1, both the FSB and RAM will run at 133MHz. For the 4/3 option, the FSB will run at 133MHz, and the RAM at 100MHz. This allows you to use non-PC133 (PC100) SDRAM in the system while maintaining a 133MHz FSB. Yous should leave this setting at 1/1, or auto, unless you run your FSB at something like 166MHz and your RAM cannot handle 166MHz.
Intel Pentium III processors simply hang when they overheat, effectively shutting themselves down.