While the new "metro" Xbox 360 dashboard is not publicly available yet, one of the touted features is the ability to store game saves and Live profile information via the clouds. Thankfully, you won't be hard-pressed for space, as you can store up to 512MB.
For the sake of comparison, PlayStation Plus subscribers get access to 150MB in game saves. If you're not familiar with the concept, it essentially means you'll be able to access your save data from other Xbox 360 consoles at your will (once you've uploaded the data, naturally).
The new dashboard has no set release date, but speculation points toward a late November release. You may might want to back up your multi-hundred hour investments sooner rather than later.