The Athlon 64 or fx53 should run cooler than a P4. Dont kid yourself. The Pentium 4's do create a lot of Heat. The new Prescott P4E's with 90 nm Die process run even hotter. I think often that this information is spread by salesmen. Salesmen should never be trusted to tell you the truth about anything. Salesmen often work on commission and the more expensive the computer the more money they make so it is better for them when you pay more money. Nothing wrong with a higher priced comptuer if you are actually getting a higher value for more money. However, don't get ripped off.
As far as speed and performance goes the Northwood P4C's with 130 nm die process that run at 800 Mhz have an edge on the Athlon XP processors. I think both of these have a tendency to run quite warm. However, I built a computer with the AMD Athlon XP2800+ processor using the Retail Heat Sink and Fan and I dont think I have ever even checked the Temperature it runs at. It has been running great for about 6 months. Don't beleive this running hot garbage.
The P4EE (Extended Edition) is a fast running processor, but I dont know if the $1,000 price tag is worth paying. Toms Hardware did an article a while back where they pointed out that a Dual Xeon Motheroboard system is cheaper than a P4EE (Extended Edition).
If you have to have a petium 4 processor the P4C processors that run at an FSB 0f 800Mhz are the best deal out there. However, the Athlon 64 bit processors are very fast and can create more throughput with the existing memory simply because they are 64 bit.
If you do not like my post feel free to point out your opinion or my errors.