MTU settings

[Mauser 98K]

any1 know how to change the MTU in WindowsXP home edition SP1?
i have heard a few diffrent things on how to do this.
i have heard that ya have to make a new key in the Hkey-local-machine\system\current controllset\services\NdisWan, called protocol with a key in ther called 0.
and add new DWORD Values called
PPPProtocolType 0x00000021
ProtocolType 0x00000800
ProtocolMTU (0x00000258 (600) or whatever i want the MTU to be)
and i have also heard it was in the Hkey-local-machine\system\current controllset\services\TCPIP\Perameters\Interfaces
with a new DWORD value called MTU.

does any1 know the correct registry key to change inorder to change the MTU to a custom setting?
need to set it lower for a more stable connection.