Almost every single source out there is saying that Xbox 360 will use 3 64 bit processors, all multi-core based on IBM's 64 Bit POWER5 architecture. Each 64 bit processor will run at about 3 Ghz, and will share 1MB of L2 cache.
It's expected to have 256MB of RAM, more than plenty for a next-gen system.
The graphics card is gonna be phenomenal. It's being created by ATI, and will not only contain a graphics rendering core but embedded DRAM acting as a buffer that will be big enough to handle a 480i image, and can be 4x over-sampled and double buffered.
http://www.engadget.com/entry/1234000600035065/