I use FreeNas that is loaded on a really old PII. I stream all my media using the SMB protocol with no issues. With that said, now your question. Any NAS or computer should be able to serve up media to boxee or the boxee box, as long as the box supports windows filesharing (aka SMB ). When purchasing a NAS, just make sure that it supports SMB. and you should be good to go.
HTPC - Boxee 0.9.22.12594, Win7, MSI AMD 785 MB w/ ATI/AMD 4200 HD IGP, Athlon II X2 250, 4, 500GB HD, Wireless G PCI, MCE Remote
Acer Aspire 1410 Netbook - Boxee 1.5, Win7 64bit, U2300 1.2GHz dual core, Intel GMA 4500M, 2 GB Ram, 160 GB HD, Wireless b/g/n