Xbox -> Hacking -> Hardware Upgrades -> Ram Upgrade:

Upgrading the RAM is possible with most motherboard versions (1.0-1.5) but is extremely difficult and not recommended unless you are very experienced at soldering.

How to upgrade Xbox RAM∞

Note: Xbox games are NOT capable of making use of the extra installed RAM. Some games will need to be patched before they will run at all. However, several types of applications do run better after the upgrade like Linux and emulators.