Your router has the most up to date version for Rev B routers. From what I can tell, there isn't going to be any more updates for Rev B routers. There are currently Rev E and I routers.
Does your router and QoS rules or just enable or disable?
If it has additional QoS rules, review the Gaming, Gamefuel for XBL in the
FAQ LibraryWhat is your ISP speed up and down that you pay for?
The motorola modem shouldn't be an issue unless the ISP signal going to the modem is low. Using any TV line splitters by chance?
If the 615 doesn't have the additional QoS rules for configuration, then maybe time to get into a different DIR series router, I recommend a DIR-655, 825, 645, or 857. I have each one of these routers and they work well. I also have 3x DGL-4500s which still rocks for gaming.