If your gaming PC uses Windows XP (or Windows 2003) and you mainly use it for gaming (I have a "dedicated gaming PC" only for that and watching movies/listening to music), you can change one option in the boot.ini file to achieve two goals:
- Force detection of 3 GB of RAM in XP (32bits/x86). I've seen a few systems who didn't recognized correctly the 3 GB before this tweak.
- Increase the user process address space to 3 GB. This way a single application can have as much as 3 GB for itself. This is great for videogames who eat almost all your PC resources.
Note: The game's exe must have been compiled with a flag to be "3GB-aware", so not all games benefit from this.
The command to place on your boot.ini is "/3GB".
Another common tweaks are disabling DEP/NoExecute (command "/noexecute=AlwaysOff"), moving your pagefile.sys file to another hard disk (instead of the one in which Windows is installed), or disabling pagefile at all if you have 3 or 4 GigaBytes of RAM (4 for Vista, 3 for XP).
If you have Vista you can also fully disable Aero and disable the "Desktop Window Manager Session Manager" and "Superfetch" Services. Also, if you don't use them, disable the System Restore Points as they have a significant impact on HDD activity.
More info of Memory under Windows and of DEP.
Disclaimer: As I said, this is optimum for gaming only pcs. If you use your pc for working, development or other tasks it is not recommended for example to disable Superfetch.