Microsoft's Andre 'Ozymandias' Vrignaud is asking some interesting, if Microsoft-centric questions, such as how much memory is reserved for OS on PlayStation 3.
So, apparently, all of the Xbox 360 system information (dashboard/online) fits into 32mb, resident at all times. Pete Isensee at Microsoft comments of the alternative: "For comparison, PS3 developers have to deal with a system that has memory split into two 256MB banks, one of which is reserved for graphics only. A large portion of both the memory banks are reserved for the system. On top of that, games that want to support other features, like friends lists or in-game commerce, take an additional memory hit."