I have built a few NAS-like systems here and there, they're possible but have niggles, you indeed need a CPU with quite a bit of power, you can do software RAID, but going for a RAID card is also an option they also reduce the load on the CPU, with a PC based NAS you can invest into a pair of high end LAN cards which will speed up data transfer.
Been a while I built something like it, at the moment my brother and I run an off the shelf NAS, performance is good, it works as it should.
As for my (for now) final vintage build, so one of the three MSI 6526GL boards actually works, I have a fitting PSU and an old Compaq Deskpro casing, actually needed the cut out the rear opening a bit so the power socket and on/off switch would fit, silly Compaq..
Specs, that mainboard has the 845G/GL/GE/GV/

chipset, means it also doesn't do a 533Mhz front bus so CPU's were a tad limited, I have some slow P4's fastest being 2Ghz, I have a 2.4Ghz P4 but that one needs the 533Mhz bus so I decided to use the fastest Northwood Celeron I have, it's running at 2.6Ghz, for cooling I have done things quite different...
The backplate of Socket 478 and of Socket 939, yes, that's AMD 939, are the same so I took the backplate of a Medion machine and mounted the socket 939 cooler mounts to that, fitted a Scythe Ninja Mini with a 92x92x25mm fan, so a modern heatpipe cooler with a rather large fan, the machine also has a 92x92x25mm intake and exhaust fan which run at a moderate speed, haven't seen a Netburst CPU run this cool, 36c max so far which is pretty damn cool for these chips.

The mainboard has no AGP slots but the on-board is working well enough no complaints there, only two memory slots, fitted 512MB RAM minus the 8MB for the graphics cars is still enough for Windows XP.
Drives, one Maxtor 250GB PATA drive, and I found an old Aopen DVD drive so the even older CD-ROM will go to one of my other machines which lacks a optical drive, will be MUCH easier to get drivers etc onto it then.