does the slingbox have different settings when its going external? (i know my plex does).
While it is dual band.. the wireless is still not great overall on the CNG3 (better than the others.. but still not great).
My previous router of a 10/100 port Linksys E2500 dual band had better range.
Your BEST overall solution is to bridge the CGN3 as well then.
(there are other advantages to the CGN3 though as well, with the 24 channel capability, etc.)