From this site:
What do you think about it?Myth - "Adjusting the SecondLevelDataCache Registry value to match your CPU's L2 Cache size will improve performance."
Reality - "If you do not set this registry value, Windows will use the HAL (Hardware Abstraction Layer) for retrieval of the L2 cache size automatically on boot up. If this fails, a default value of 256KB is used. This is where the myth that XP can only use 256KB of L2 cache comes from. But it's false. The SecondLevelDataCache is designed as a secondary source of cache size information for computers on which the HAL cannot detect the L2 cache. The HAL is able to retrieve the L2 cache size from any CPU using the set-associative cache design (Pentium II or newer), it's only the relatively few older processors with direct-mapped L2 caches that the HAL will not recognize. This tweak only applies to older computers (Pentium I or older) with over 64MB of memory. On these older systems, this tweak can improve performance by about 0.4%. Pentium II and later CPUs are not affected." Source
I just changed my value to 512 but I've not seen any difference.