Why not do it the fun/cheap/easy way?
Get a decently-powered computer, like a P2/500+ with a bit of ram in it. (something that can handle the traffic)
Buy 2x "4-port NIC" cards (look like a reguar NIC, except there's 4 RJ45 jacks on the back, they act like 4 seperate NICs in 1 card)
Plug 6 of them into the various DSL modems, 7th into your LAN, and 8th "for future upgrades?"
Then download and install
Smoothwall or similar router OS software, make sure it supports "connection round-robining", so that for every new connection, it'll use a different DSL line. (Note: this may not work with some programs/games due to the fact that your computer is sending/recieving from 6 different IP addresses
)
Edit: after checking the smoothwall manual, i'm not 100% sure smoothwall supports connection round-robining...
I installed it once, and I'm pretty sure there was a way to add more than one "RED" (internet) NIC.