More than likely the sales guy you speak with won't be able to tell you too much about the CPU. AT least, that is the case for most mail order shops. Your best bet, if you want to make sure you can overclock is to by a pretested TBird. I do not know that many places that handles these, but a quick searc on the net should turn up quite a few. Hopefully others will chime in as well with suggestions.
Beyond that, you could just get a Duron as they tend to be easy to overclock and are more overclockable. You are not going to get too much of a gain OCing a TBird 900 or 950, whereas you can get a 50% gain with most Duron 600s. As for color and week codes, they are not the most reliable IDs of a chips OC potential.
One last thing to consider, if you do get a Pro2A, you can always unlock a 'locked' CPU with a pencil by closing the L1 bridges!