The Tbirds come in two types.
Blue:
These TBirds are made in the new plant in Germany using the new copper technology. These CPUs are the ones that are used for the 1.1GHz and 1.2GHz TBirds. They are also found in some of the other TBirds as well. Since these CPUs are made using the new technology, they are able to go faster. This is why you should be happy if your CPU is blue.
Green:
These TBirds are made in the older plant in Austin, Texas. They are made using the older aluminum technology and have a lower top speed. They are used for TBirds up to 1GHz. They are not used for the faster TBirds since they generally do not go that fast (yields low). You can still get these above 1GHz, but chances are better with a blue one.
The way you can tell is if you hold the CPU at an angle, the blue ones are blue, the green ones are green. I have never seen a blue one myself, but I hear it could be tough to tell without something to use as a comparison.
The week number is also important. Earlier week AMD chips tend to overclock less than the newer ones. For example the week 22 Duron 600s generally got up to only 800 or 850. The week 36 Duron 600s generallu get up to around 1GHz. More specifics on this can be found at
www.ocworkbench.com.