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*