DDR-3? LoL... jumping the gun, aren't we? :giddy:
Now I don't know much about X-box 360 hardware, but here's my thoughts....
DDR-3 is used exclusively for graphics cards, at the moment. Not only would upgrading your graphics card memory do absolutely nothing (because games are optimized to run utilizing only the amount of memory that shipped with your console), but upgrading your graphics memory will probably be an impossibility.
If you're referring to system memory, then you must mean DDR-1 or DDR-2, but I'm pretty sure consoles like X-box don't really need system memory, it's all used for graphics. Upgrading system memory, if it is an option, would be useless as well.