Probably the trickiest concept when comparing the Tri-Core XBox 360 to the PS3 (technically a single core) processor is understanding the "Cell" processors that the PS3 uses. Technically the PS3 is a High speed PPC processor with 8 Cell co-processors .. of which 6 are available for general gaming use. (I don't have the link, and will dig it up .. but I read one cell is dedicated to the Sony OS and the other is a "spare" used to take the place of the other 7 cells should a defect occur).
Wikipedia has a good overview of the cell processor with a picture of the die. (http://en.wikipedia.org/wiki/Cell_(microprocessor)).
Here is an a great overview document explaining the cell architecture by a guy named Nicholas Blachford. Its http://www.blachford.info/computer/Cell/Cell0_v2.html
IBM Research page has two pages discussing the broadband cell processor and its architecture.