Normally, the tweak I've seen asks you to go into:

1. HKLM\System\CurrentControlSet\Control\Session Manager\Memory Management
    
2. And change the value to either "0" or "1" to the adjustment the "LargeSystemCache.".

However, in Windows XP, all you have to do is:

1. Right click My Computer
    
2. Select Properties
    
3. Click Advanced
    
4. Choose Performance
    
5. Click Advanced again
    
6. Select either Programs or System Cache under Memory Usage.

Programs = 0 for the registry tweak equivalent
System Cache = 1 for the registry tweak equivalent

From arstechnica.com:

On NT Server (in this case XP), the Large System Cache option is enabled, but disabled on Workstation. The two different settings effect how the cache manager allocates free memory. If the Large Cache option is on, the manager marks all the free memory, which isn't being used by the system and/or applications, as freely available for disk caching.

On the flip-side (with a small cache), the manager instead only sets aside 4MB of memory for disk caching in an attempt to accelerate the launch of applications. Or in a more technical approach, if enabled the system will favor system-cache working sets over process working sets (with a working set basically being the memory used by components of a process).