One of the biggest differences is that there is no user mode and kernel mode on the xbox. Basically when your game boots up it is running in kernel mode with access to everything. You game must in essence act like the operating system for the console while it''s running. You gotta fit your program into 64mb of ram and watch out for trashing the memory, there''s no paging on the xbox unless you implement it. You do have a certain amount of space your game is allowed to take up on the xbox hard drive as well while it executes. It think the XBox will preserve this buffer for the last 4-5 different xbox games played. This is different the the perminant save game area which will persist on the HD indefinitely. Depending on how well you wrote your code and how abstracted it is, I can see you porting your game to the xbox in 3 months. I think the bungie guys has a working version of halo ported in 3 days...

The biggest issues will be:
1) Memory - 64 MB is not a lot. You probably dont want any dynamic allocation of memory at runtime
2) Resolution - Your game needs to look good at 640 480
3) Control system - the XBox controller has 2 analogue sticks and a unique button layout. You need to design your game to fit around this control system.

And of course, you need to physically get hold of a Dev Kit, they might not be as readily available as you think.

http://www.positech.co.uk

I work for microsoft, but not in a game dev capacity. I''ve attended a few infromational meetings about the Xbox however. Microsoft provides a wide array for development tools and APIs for working on the xbox. Just about any of your common funcitons like the ones you specify have an xbox equivilant or are identically implemented. The xbox runs on a stripped down version of the win2k kernel.