The first launch Xbox 360s had the Zephyrs chip set, the CPU and GPU were both 90nm size chips

Then they revised it to the Falcon chip set in 2007, where the CPU was now 65nm size.

Now, they did it again in 2008, with the Jasper chip set were now both the CPU and GPU are 65nm size. You can find out if you have it if your Xbox was made after Sept 08-2008 Lot 8031 and the Team was CSON.

But Microsoft is not done yet, it is expected that a fourth chip set will be produce called the Valhalla which will combine both CPU and GPU into a single chip. This is expected in 2009/2010
References :
http://tech.blorge.com/Structure:%20/2008/05/09/microsoft-prepares-for-new-xbox-360s-65nm-gpu-jasper-chipset/
http://www.product-reviews.net/2008/09/30/xbox-360-with-65nm-jasper-chipset-in-stores/

If you would like to play XBL send post your GT
References :
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