Xbox 360 reserves 32 MB for the system. The remaining 480 MB (94%) is completely available for games to use how they wish. The 32MB of system memory is used for the kernel, device drivers and the Xbox 360 Guide, which includes friends lists, voice/text messages, achievement lists, gamercards, Live Marketplace, 1-1 chat, virtual keyboard, and the music player. For CPU reservation, core 0 is completely available to the game, and the system reserves a small percentage (think single digits) of core 1 and 2.
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. Arstechnica.com indicates that a total of 96MB is reserved for the system on PS3. Innerbits.com indicates that 9MB is required for friends lists (and 60MB for in-game commerce!). If those numbers are correct, a PS3 title using friends lists functionality has 512-96-9 = 407MB available, 73MB less than an Xbox 360 title using the same features.
Memory Reservation on Xbox360 and PS3
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