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Xbox 360 S
Technically designated the Xbox 360 S and marketed simply as the Xbox 360; current Xbox 360 consoles are based around a redesign of the Xbox 360 hardware which was officially announced on June 14, 2010 during a press briefing prior to that year's E3.
It was speculated that a complete redesign of the Xbox 360 hardware was being produced after pictures of a possible new motherboard design surfaced on March 17, 2010. Ads later surfaced on June 13, 2010 showing a slimmer Xbox 360 design, which was expected to include a 250 GB hard drive and integrated Wi-Fi functionality.
Xbox 360 S consoles feature redesigned internal architecture with the Valhalla motherboard, which allows for around 30% more space than previous motherboards, and the XCGPU, an integrated CPU/GPU/eDRAM chip using a 45 nm fabrication process. This allows them to be both smaller and quieter than the previous versions of the Xbox 360. They also feature 5 standard USB 2.0 ports (2 more than previous models) and an additional custom USB port for use with peripherals such as the Kinect sensor. Unlike older models, 2.4 GHz 802.11 b/g/n Wi-Fi and a TOSLINK S/PDIF optical audio connector are also built-in, allowing for digital audio and wireless networking without the need for external adapters. The Memory Unit slots found on previous consoles have been removed in favor of the USB flash drive solution added in a previous system software update (released on April 6, 2010) and the power and DVD drive eject 'buttons' are touch sensitive rather than the physical buttons found on previous models. The external hard disk drive connector has also been swapped for an internal bay for use with a proprietary hard drive; this is neither a standard drive or compatible with older models. Other notable hardware changes include the use of one larger fan compared to the previous Xbox 360 models (which used two smaller ones) and the design of the vents, which are similar to those used on the original Xbox. Unlike previous generations of the console which had names to distinguish different SKUs, the new models are to be marketed solely by the amount of included storage, in a similar fashion to current models of its main competitor the PlayStation 3. When the first new models began to ship, remaining stock of the Elite package dropped in price to US$249.99 or AU$349 and the Arcade dropped to US$149.99.
Xbox 360 250 GB
The first Xbox 360 S SKU to be revealed includes a 250 GB hard drive and its casing features a glossy black finish. It was shipped to US retailers the same day it was announced (June 14, 2010) and went on sale later that week. It was released in Australia on July 1, 2010, in New Zealand on July 8, 2010 and in Europe on July 16, 2010. It retails at US$299.99, £199.99, AU$449.00, NZ$499.00 or €249.00, replacing the Xbox 360 Elite at that price point.
Xbox 360 4 GB
A second SKU which includes 4 GB of internal flash storage and has matte black casing (much like the Xbox 360 Elite) was released on August 3, 2010 in the US and August 20, 2010 in the Europe. It replaced the Xbox 360 Arcade and is priced at US$199.99, £149.99 or €199.99. Although this model has on-board storage, Xbox Product Director Aaron Greenberg confirmed that it does have a drive bay which Microsoft has "the opportunity to use in the future". On August 20, 2010, Microsoft announced a 250 GB stand-alone hard drive for use with Xbox 360 S models priced at US$129.99
Source: Wikipedia, "Xbox 360", available under the CC-BY-SA License.