If all you know is that you're trying to guess a number between 325 and 450, then every number is equally likely and any guess is as good as any other.
But, is this an actual bet? Is something important (lunch, or public humiliation) riding on the outcome? then if I was your colleague and I had 325 CDs I'd give you the 325-450 range, knowing that most people would guess the middle number. But you'd know that, and guess something far away from the midpoint.. But he'd know that, and.....
...at this point you'd want to consult a game theory text.