There's no need to guess - Toms done the tests
In the particular configurations and tests they did - the P4 2.8GHz is substantially faster (up to 25%) than the Athlon 2800XP - except for one test, video encoding.
And what does it prove? - nothing at all, except that they're different and have different strengths.
And the prices - just checked on ebuyer and the P4 is more expensive by a small margin £265 v 247 (ex VAT - latest boxed versions of each).
So pleeeese no more "Athlons much cheaper and much faster than P4". In fact the P6 2.67GHz is more comparable in performance to the 2800XP, and it's a fair bit cheaper too (£175 + vAT)