Well after some talking on irc i relised the easyist way to do it, and ive now done that
It is as follows:
Leave the main router to DHCP.
Disable the second router's DHCP.
Manualy setup network settings for any computers that are going to use the second router's internet connection.
So my download server has manualy setup settings and other machines auto get a address from the main router and use that connection.
/me is pleased
I also drew a quick network diag. My network looks something like this some of the time
Thx for the help.