One question: were you talking about the 2.5 GHz P4 on a 400 MHz fsb, or the 2.53 GHz P4 on a 533 MHz fsb?
Most people cannot notice a speed difference unless chips are at least 10% apart. The 2.8 GHz P4 is just about 10% faster than the 2.53 GHz P4 on a 533 MHz fsb. So you may or may not be able to sense a tiny speed difference. In that case, due to the extreme price difference, I'd get the 2.53 GHz P4 (Actually I'd get the 2.4 GHz P4 on a 533 MHz fsb and upgrade later, but that is since I'm really cheap).
The 2.5 GHz P4 on a 400 MHz fsb is beginning to be held back by its slower fsb speed. In that case, I think you would definately notice a small speed difference between it and the 2.8 GHz P4. Plus your upgrade path would be blocked since I don't think Intel will go much farther with the 400 MHz fsb chips.