Pentium 4 3 GHz vs. Atom 330

cpubenchmark.net/Atom+330 @ 1.60GHz
ark.intel.com/Product.aspx?id=35641
tomshardware.com/forum/261282-28-better-setup-atom-3ghz
Bajo los tests del Everest (CPU, FPU, etc) queda más o menos entre un 
Pentium 4 EE 3,6 y un 
AMD X2 4000+

Go with the Atom n330 for low power and silence. The P4 for more grunt in single threaded applications. The atom would be about the same is heavily multithreaded apps but slower in single threaded apps.

The Atom 330 will almost certainly not have the raw processing power of the P4, but it won't be a slouch for the purposes you've described either. The amount of RAM and the speed of the HD will be much more important. If you can afford it, get the Atom system with the 4GB RAM, use an Intel SSD for your OS and swap file and a large 7200RPM HDD for data. This will give very good all around performance. Assuming you are using Windows 2000 or newer, also remember to disable executive paging and use a decent video solution (Nvidia 9400 or ATI 790/785G onboard, or any DX10 discrete card). 

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You can get easily passive heatsinks for a 3.8 GHz Nocona/Irwindale Xeon, but they're not really the passive heatsinks you have in mind. The ones you can easily get are ~one-pound solid copper units that are low-profile designed to be put in 1U or 2U rack servers. They use ducted airflow, baffles, and very noisy high-RPM, high-CFM fans. You may be able to find a desktop-style skyscraper-and-heatpipes passive heatsink for a 3.8 GHz Xeon but you need a huge one plus good case airflow as the CPUs throw off well over 100 watts. I put together a dual 2.8 Irwindale for a guy and used the 2U active heatsinks (a one-pound block of copper with a 5500-rpm fan) and those put off some serious heat. I would hate to see what the 3.8s would do.

List of CPU power dissipation

Atom 330

Diamondville (45 nm)

1.6 GHz

8 W

5.00 [2.50 Per Core]