Reading the multi-wan forum would be helpful: http://forum.pfsense.org/index.php/board,21.0.html
Also from the documentation: http://doc.pfsense.org/index.php/Multi_WAN_/_Load_Balancing
Basically any old machine you have lying around will be able to handle your bandwidth. The main load on these machines would be if you have multiple incoming VPN connections (i.e. employees, remote workers, etc.), traffic shaping, squid proxy, snort, etc. Your config probably won't need anything fancy. Just make sure the box has three network cards and you should be good to go.