HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PriorityControl\Win32PrioritySeparation
The suggestion was to change this value from 2 (default) to 26 to increase the processor time devoted to the foreground application, subtracting it from the time devoted to background tasks (= good for FS2004; bad for a web server).
BUT...
I found another web site, devoted to demystifying myths about tweaks, which said that basically this operation is useless because that 26 value is already the default, and can be easily changed with Control Panel-System-Advanced-Performance -Settings-Advanced-Processor Scheduling - Programs (this btw is the default setting in Windows XP Pro).
WELL
I opened the aforementioned Control Panel applet and the setting was the default one (optimize for PROGRAMS). BY THE WAY the above mentioned registry key was set to "2".
SO
I just clicked PROGRAMS again in the Control Panel and clicked APPLY (that is, I reapplied the setting which was supposedly already in place).
And sure enough, now the registry key read "26"!!!

I will check if I notice any performance gain in FS2004; anyway I'm running Windows XP SP2 and I never messed with that registry key before, so it's likely to be a bug.
What is YOUR registry key reading btw? 2 or 26?