Yep, it can be set on both side of the interface.  I can't remember clearly, but there was a performance increase when both sides were explicitly set to 100Mbps Full Duplex and not allowing either side to auto-negotiate.

Like I said, I'll have to look into getting another Wireless router.  However, I can't understand your DMZ comment.  DMZ would allow the NAS to be seen from outside the 3800's firewall which I wouldn't want.  Unless you mean in the DMZ of the new router...  That would make more sense.

The other wireless router that I have is essentially acting as  WAP with a switch on it (behind the 3800), so I should be able to accomplish the same thing with a new router as well (turn off NAT, DHCP, etc).

Thanks for the suggestions.  Would be easier (and cheaper) if they just put the old functionality back in