sbc dsl and linksys firewall config issues

[hanasaki]

What would make double NAT fail?  fyi: works in my home just fine though 
two Linux iptables setups for something I was playing with.  Then again 
NAT on Linux may be better than that in a hardware firewall?

Hmmm what's the association with the MTU setting?  And just where is 
this set in the dsl modem and in windows?  "just works" with Linux... 
too bad this system isn't fully under my control to do "right".   How do 
you find the "right" MTU setting?