They say that the Thunderbirds with the Blue die, the square metal part protruding from the top of the cpu, are more overclock friendly.
My T-bird has a slightly greenish tint to it, therefore is not the "desired" but I just got a new Thunderbird 1100 today and it has a pink looking color to it.
I haven't tried overclocking the 1100 yet, as the 950 is in as I type this, but I haven't had any problems so far with the green looking thunderbird.
Some say the difference is so minute that you can' t tell the difference. I really don't know. Supposedly the "Blue Core" thunderbirds use copper interconnects which provide better heat transfer, but I haven't seen hard evidence of this.
If your looking for a good budget system that is overclockable without spending quite so much, you may want to consider the Duron, they are very stable and overclockable.
Good luck
Run-em Cool