A PIII 500Mhz can do 80Mbps to 90Mbps with just a single WAN connection (WAN->LAN, LAN-WAN), using packet filteirng and NAT.
(Same with a VIA C3 1Ghz…But I was using Intel NICs)
Once you start adding things like Squid, Samba and such, it drops down.
It won't be a problem for 2x 10Mbps lines.