CPU - P 2.80 HT (Northwood) is the fastet CPU because of the larger cache, higher IPC and Hyperthreading, the second fastet CPU would be the Celeron D 345 and third, the Celeron D 320

The 1.6 Ghz P4 would be slower than the Celeron Ds begin probably fourth place when the 2.0 Ghz Northwood Celeron would be the worst CPU of the list

Motherboard - The best choices are ethier the Gigabyte GA-8IG1000MK or the ASUS P4P800-VM (Both motherboards based on the Intel 865G) if you are going to do some OC, but the Intel D865GLC (also based on the Intel 865G) is also a very good canditate and good when it comes to stability at the cost of not having OC options, Only these 3 motherboars has Dual Channel DDR 400 Interface
Both VIA based motherboards (MSI PT8 Neo-V and ECS P4M800PRO-M478) won't be really good choices for OC or mem perfomance, despite VIA made some good chipsets, these ones are not the best choices when you have i865 based motherboars, the ECS mobo despite supporting DDR2, the memory controller is only single channel DDR 400/DDR2 533 compared to the Dual Channel DDR 400 interface of the i865G based motherboards (4200 MB/s with Single DDR2 533 vs 6400 MB/s on dual DDR400 (Theorical max bandwitdh) plus DDR2 has generally higher latency than DDR1). The MSI PT8 Neo-V uses the older VIA PT800 which is a good single channel P4 chipset, but not fast enough to compete with the Intel 865G/PE chipset, the VIA PT800 would compete better with the SiS 648FX or the Intel 848P

PSU: It really depends, i don't have really much experience with PSUs

Concur with Carlos's assessment of the CPUs and Motherboards.

Any of the PSU's I believe should be fine unless you loads it up with a ton of other internal components. If you keep it to a Optical Drive, Hard Drive, and a GPU I would assume 300 Watt should be fine. The only question would be what is the quality of the PSUs etc which they look like mostly middle of the road from that perspective.

Also i have alternative set of CPU and Motherboard on Socket 939:

1AMD Athlon 64 3200+ - ADA3200DAA4BP (ADA3200BPBOX)
2http://www.cpu-world.com/CPUs/K8/AMD-Athlon%2064%203200%2B%20-%20ADA3200DAA4BP%20(ADA3200BPBOX).html
3Foxconn 6150K8MD-8EKRSH Socket 939
4http://www.foxconnchannel.com/ProductDetail.aspx?T=Motherboard&U=en-us0000157

Is it really more powerful, compared to this configuration from the first post? (if ignore slots for video cards, having the same DDR RAM and PSU in both motherboards)

1Intel® Pentium® 4 Processor supporting HT Technology 2.80 GHz, 512K Cache, 800 MHz FSB
2ASUS P4P800-VM

+1 for the CPU and motherboard choice. I just want to add that the AcBel looks like a server grade PSU so should be built better than the other ones. Also it is 80% efficient according to the spec sheets and has enough wattage that you should not have to worry about power draw.

The 939 Foxconn motherboard can support dual core processors and newer, faster, better PCIE video cards. The performance of the Athlon 3200+ should be similar to the P4