Ralf: the core-family Celerons are pretty cheap: Celeron Conroe-L Model 430 (1.8GHz, 512KB cache, NX bit, x86-64, 35W?) pops in at around $60, that's the same prices as a Sempron LE-1150 (2.0GHz, 256KB cache, NX bit, x86-64, 45W).
Given the lower power consumption and larger cache size, I'd definitely go with the Intel CPU - and the motherboards aren't really that much more expensive, plus you get a nice intel chipset there.
2GB of RAM might seem like a lot, but it means you can have stuff cached and wear the disks a little less. But considering that 2x1024meg and 2x512meg DDR2-800 costs just about the same, ~$100, I'd opt for two gigs.
Sure, that box might be a little overpowered for a NAS, but it has the advantage of being a fully-fledged computer, which I'd also use for
torrents (instead of having my power-hungry desktop on all the time),
subversion, and the occasional linux testdev.
Sure, probably consumes more power than some of the NAS boxes, but the early synology NAS were powered by Pentium4's afaik, so... *shrug*
