(and thanks to Timxirish for the extra information and pic's.)
Xbox 360 Console Revisions Information

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Console Amp Ratings

Take a quick look at the back of your console. Depending on your revision, the information could be on the right or left side. You are looking for a small bit of information which says "12V--X.XXA". The following shows which revision you have by simply checking the 12V rating.

Xenon = 12V--16.5A
Zephyr = 12V--16.5A
Falcon = 12V--14.2A
Jasper = 12V--12.1A

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The Revisions




Xenon
The original Xbox 360 configuration used in the initial Premium and Core machines released in the end of November 2005. These are also know as the RRoD (Red Ring of Death) machines because the GPU chip warps away from the motherboard because of excessive heat.

* IBM 90 nm CPU
* ATI 90 nm GPU and 90nm on-chip eDRAM
* Low profile GPU cooler
* Standard CPU cooler
* 203 Watt output power supply
* Original four-pin fan connector
* 16.5A on back sticker

Note: This is the original console which is the most prone to the RROD problems, as well as numerous other overheating problems.

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Zephyr
Zephyr was the first revision with HDMI connector and supposedly fixed the RRoD problem cause by the inadequate GPU cooling on Xenon boards.
It was introduced with the Xbox 360 Elite in May 2007. In July 2007 Xbox 360 Premium machines began appearing with the Zephyr motherboard.

* New Elite model includes 120 GB hard drive
* Updated motherboard layout
* HDMI output
* Glue around the CPU
* Extended GPU cooler
* Still 90 nm CPU and 90 nm GPU/eDRAM
* Customized three-pin fan connector; Middle pin is a shared ground for the exhaust fans
* 16.5A on back sticker

This revisions is also very prone to the RROD, even though it is claimed to have fixed the problem. This version is still not the best to have, but does have HDMI.

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Falcon
All Xbox 360 Premium machines and Arcade machines manufactured August 2007 introduced the new 65 nm CPU accompanied with a new cooler and still 90 nm GPU with the Zephyr cooler. The motherboard is based on Zephyr and requires fewer components (some capacitors and coils removed) for the new 65 nm CPU, resulting in lower costs.
Starting September 2007 the newest Xbox 360 Premium machines shipped with the Falcon board. Xbox 360 Elite machines were eventually updated to Falcon boards too. Xbox 360 packages with LOT number 0734 or higher will most likely be Falcon machines.

* IBM 65 nm CPU
* ATI 90 nm GPU with 80nm on-chip eDRAM
* Bigger and enhanced CPU cooler
* Fewer voltage regulators, capacitors and inductors
* 175 Watt power supply, 12V rated for 14.2A, new connector
* Same fan connection as Xephyr

This versions is not as prone as the rest of the first revisions, but have been known to still have the RROD, but there is a smaller amount of consoles with the error.

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Opus
This is an intermediate motherboard with 65 nm CPU and 90 nm GPU, new coolers, but no HDMI. It will be used in Xenon chassis for repairs and RRoD affected machines.
Started to appear end of July 2008 in machines coming back from repair.

* Falcon based board
* No HDMI output
* IBM 65 nm CPU
* ATI 90 nm GPU with 80nm on-chip eDRAM
* Falcon CPU cooler
* Zephyr GPU cooler

Like said above, this revision is a simplified revision of different parts which Microsoft used to repair the original Xenon machines.

This version is not able to be purchased in stores, as it is strictly a repair console.

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Jasper
Jasper was released in late 2008 and cropped up in Arcade machines at the end of November. It introduced the 65 nm GPU to reduce power usage and cost further. The new powersupply is rated for 150 Watt, down form 175 Watt on the Falcon. The Amperage on the 12V rail is down to 12.1A (12.1A x 12V = 145.2W).
The on-board flash has also increased to allow the new NXE (New Xbox Experience) dashboard update to reside internally, instead of requiring a memory unit or the hard drive.

* IBM 65 nm CPU
* ATI 65 nm GPU with 80 nm on-chip eDRAM
* 150 Watt power supply, 12V rated for 12.1A, new connector
* The new 150 Watt power supply is slightly smaller than the older versions
* Larger 256 MB on-board flash, up from 16 MB, to accommodate the NXE update
* On-board flash memory only on Arcade Console, other versions have an HDD
* Same fan connection as Xephyr

This is the newest and best console to have at this point. It has both 65nm chips which reduce heat in a large amount, as well as onboard memory.

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?
Valhalla
A future version that will have both the CPU and GPU on a single die. Slated for 2009.
This is the next version that is currently in the works. It is supposed to solve the problems 100% and have both the chips on one die. There is not much information about this console, but the Jasper revision is the only known solution so far until Valhalla is released, if it is released.

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Current Revision in Arcades: Jasper
Current Revision in most Elites: Jasper
Current Revision in most Pro/Premiums: Jasper

There are numerous reports of Jasper's appearing more each day other consoles besides Arcade, but there is almost a 100% chance that getting an Arcade will get you a Jasper.

UPDATE: Pretty much every brand new Retail box of an Xbox 360 console will be a Jasper now.

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How to get a Jasper Console?

In order to be 100% certain when purchasing a new Xbox 360 Console, check on the side of the box where the serial number of the console is that the store scans. Slightly push the Console down and to the left in order to view the writing on the console. You are looking for this:
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12.1A

If you see this, then it is 100% a Jasper revision.

The following is how to easily check which revision is what:
12.1A = Jasper
14.2A = Falcon
16.5 = Xephyr/Xenon


I hope this all helps you guys and makes it much easier in understanding the differences and what to look for if you want a console that won't fail.

Please do not think that all consoles out in stores are the newest revisions. Stores like Best Buy, Wal-Mart, etc sometimes have old stock, and will continue to sell the stock until it is gone.

When going for a new console, be sure to be cautious, and remember this thread, because it will help you a whole lot and save you from sending in the console three, six, ten, twelve months from now.

Better safe, than sorry.

(and thanks to Timxirish for the extra information and some pic's.)