The "DisablePagingExecutive" entry in the registry prevents the kernel (the core of the XP OS) from being rolled out to a page file on disk. The effect of this part of the tweak is to cause the OS to cache the Kernel and its entourage to RAM instead of to disk, which makes XP far more responsive.
The "LargeSystemCache" registry entry forces XP to allocate all but 4MB of system memory, that is system memory, not available RAM, to the file system cache. The remaining 4MB of system memory is used for disk caching, though XP will allocate more memory if it is needed.
A modern hard disk will transfer sequential data to and from disk at up to 40MB per second, or even faster on some of the more expensive drives, but the LargeSystemCache tweak means that effective transfer speeds of 1GB per second or more can be obtained, depending on the amount of RAM in your system and its operating speed. This is achieved because the LargeSystemCache modification causes the OS to store data in RAM after it is read from disk. It means that the OS is always using the optimum amount of RAM instead of leaving it untouched for future use that may or may not occur. Without this part of the tweak, 200MB or more of RAM in a typical 512MB machine goes completely unused.
Some I/O intensive applications may take a hit in performance from changing the LargeSystemCache, so this particular component of the tweak should not be applied to a system that is running either SQL Server or Internet Information Server (IIS) because both of those applications perform their own caching.