A 1.6GHz Athlon XP 1900+ offers the performance of a 2.0GHz P4, so no, there is not much difference between the XP 1900+ and P4 2.2A.
Now, if you buy the P4 1.6A or 1.8A and overclock it to 2600+MHz...and mate it with a top of the line graphics card, then yes, you will tell a difference (according to the reports on other forums). However, even then, the difference will be far from massive; it will be nothing at all like "night and day."
Personally, I think the $150 P4 1.6A overclocked to 2400-2600MHz, mated with the $115 Asus P4B266 motherboard, is the best price/performance available today. That said, all my current systems use the Athlon.