For the NSM box, I'm considering a Shuttle SB52G (support, review) with Intel 845VG chipset and FB52 motherboard sold by ExcaliburPC, NewEgg, and Knowledge MicroExpress. Crucial sells memory. Other options include the Slimpro 1BayPC (manufacturer?), LittlePC, MicroPC4 and Lex Light, For more information there's the mailing.freebsd.small list, the books Embedded FreeBSD Cookbook and Designing Embedded Hardware, or Slashdot.
One issue with these small form factor devices is having enough interfaces for serving as a firewall or router. Luckily FreeBSD 5.x supports the Linksys USB100TX and USB200M USB NICs. Iomega and others make USB floppy drives. One could always buy a full-fledged but cheap PC from TigerDirect.