The original source for these specifications was a Team Xbox post which was later removed again (Microsoft didn’t like the specs revealed, I wager).

Here goes:

Xbox 360 Hardware specifications

Custom IBM PowerPC-based CPU
- 3 symmetrical cores at 3.2 GHz each
- 2 hardware threads per core
- 1 VMX-128 vector unit per core
- 1 MB L2 cache

CPU Game Math Performance
- 9 billion dots per second

Custom ATI Graphics Processor
- 500 MNz
- 10 MB embedded DRAM
- 48-way parallel floating-point shader pipelines
- unified shader architecture

Memory
-512 MB GDDR3 RAM
- 700 MNz DDR

Memory Bandwidth
- 22.4 GB/s memory interface bus bandwidth
- 256 GB/s memory bandwidth to EDRAM
- 21.6 GB/s frontside bus

Harddrive
- Detachable 20GB drive

Audio
- Mulitchannel surround sound output
- Supports 48khz 16-bit audio
- 320 independent decompression channels
- 32 bit processing
- 256+ audio channels

DVD drive
- Support for DVD-video, DVD-Rom, DVD-R/RW, CD-DA, CD-Rom, CD-R, CD-RW, WMA CD, MP3 cd, JPEG photo CD

Screen resolution
- All games supported at 16:9, 720p and 1080i, anti-aliasing

Connections
- 3 USB 2.0 ports
- Support for 4 wireless controllers
- Wi-Fi ready

Customizable face plates to change appearance:

Picture source: ourcolony.net