My Athlon X2 with an nVidia video card drew 190 watts peak and idled at 160 watts. My quad core Core 2 with Intel X4500 peaks at 60 watts and idles at 40. My Revo media PC with nVidia Ion peaks at 20 watts and idles at 5.
Sleep didn't really work well with the Athlon, so it basically ran 24/7. That put it's power usage at ~115kwh/month. At the California rate of $.037/kwh, it cost ~$43/month to run. Sleep works much better on my Core 2, so it run ~16 hours a day. That put's it's power usage at ~24kwh/month costing ~$9. That gives me a savings of $34/month. Everything in the machine is faster, including the video, and the system cost only $500. That means replacing the machine took 14 months to pay for itself. Now, I realize that some place pay a third for electricity than we do, but even then a P4 would easily pay for it's own replacement. It would just take a few years instead of 1.
The Revo's atom is as fast as a P4, and the video is better than anything that came in a P4. Even if your in one of those amazing places that sell you electricity at $0.10/kwh, it would take in the range of a year to replace the P4.
My wife's brand new laptop with an i5 draws 30-50 watts, and only cost $700. I have no idea where you spend $2k on computers. Even most Macs don't cost $2k.
Edit:
OK, I reread your post. Your comparing apples to oranges. You don't replace a P4 with a high end PC. You replace a P4 with a $200 net top. You will get equal performance, and a massive drop in electric usage. Why would you feel the need to buy a high end computer if a P4 is even usable? A $500 PC will give you massive speed ups over a P4.
Well, there's part of it- Cali electric rates
I will buy another high end computer here whenever they can demonstrate that it 1.) Will run games that I want to play that I can't already on a PS3 or 360 (so far that's a negative--WOW doesnt "wow" me and SC2 doesn't interest me either) 2.) is a RADICAL departure from current methodology or 3.) makes VIM telepathically controlled. Oh, and 4) My P4 blows up. It is my experience that without programs that make use of 64bit arches and OSes that do the same, the flasgship of 32bit x86 has less compatibility issues, runs more software, and has already paid for itself. Now, if I was an artsy-fartsy "film and picture editor" the answer would be simple-buy that 2 grand apple cinema display and a new MacPro. That would run arounf 4 grand. If I was a SC2 whore, I'd spend right around 2 grand for a mobo, phenom 2 Black, as much ram as I could stuff in, the fastest SSD, Realforce Torpe keyboard....Hell the keyboard is 260...(as much as the new Dell I have downstairs.)
