A 2500 XP overclocked to 3200 would not run as fast as a 3200 64 because the 64-bit Processors are a new line and more updated. But it would be faster than a 3000 XP.
It would be technically the same speed - but the difference in efficiency, age, technology and Cache especially would make it physically not as fast, quite likely by a reasonably large boundary.
You'd have to get a 64-bit Motherboard? But yeah; you could overclock one. You'd have to buy something more than a 2500 though; probably a 3000, but it would be far more expensive.