soekris has some interesting stuff. I'm not sure what kind of processing power you would need, they are low power 486/586 class processors. you could get something like the net5501 (http://www.soekris.com/net5501.htm) with a case that has room for a 2.5" SATA drive and 1 PCI slot that you could use for a wifi card. I've only seen them used with BSD but there is documentation out there that explains how to get linux working on them.