This is a very interesting question and one that I have been wonder about myself.
I think that the Intel Pentium 3 generally preforms better at the same speed then T-Bird. To be more clear, in GENERAL a Pentium 3 1GHz system will produce better frame rates across the board than a T-Bird 1GHz system (consider systems have same video card, etc).
I am getting a T-Bird because of price and raw MHz. The T-Bird goes to 1.2 GHz and P3 only 1GHz.