I would buy tha Athlon64 (Socket 939) because later you will be able to put dual-core AMD processors into the Socket 939. So maybe 1,5-2 years from now you can buy some more RAM and a new processor to upgrade your machine.
I actually don't know about the P4, whether the sockets of the processors will change in the future with the advent of dual-core processors or not.
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AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28