I would think it would be a fairly noticeable improvement but instead of wasting money on upgrading an old P4 rig, you are probably better off saving up for a new one.
Your comment is appreciated, however as a 76 yo cancer survivor I think twice about buying green bananas. If I knew exactly which cpu I have I would be more comfortable about replacing it. I guess all the specs are on the back which I've not seen. All my components were purchased based on the recommendations in the Thompson's first book and I paid a tech to put it together and load 98se. Have had no issues with it since early 2002.