A good guideline for adding mods is to keep in mind the xbox oly has 64Mb ram.
The game itself takes up quite a sizeable chunk of this, so any additional content you throw in there will slowly use up what little ram is left. also unlike a PC, the xbox doesn't have a page file/Swap space, so when it's out of memory, it's just plain out of memory and gives DDEs.
Three things to keep in mind when tossing in mods:
1. Anything that adds new textures and/or meshes, will use quite a bit of ram.
2. Any mods that are Just a single ESP/ESM will use next to none.
3. My golden rule: When adding a bunch of mods at once, add ONE, test it and make sure it doesnt screw up quests and stuff. add another and repeat as nessesary. trust me, it will save you a lot of hassle in the long run.
And a final note, if you do add or remove mods frequently, don't keep using the same save, it will eventually get so full of old refs that it will just not work very well.
EDIT: spelling