Can i use a new and better router with my existing dsl combo modem / router?

Just an option for you but rather than adding a "repeater" add a full access point. You simply add an "N" access point to the existing router and you have "N" coverage as well as the ability to place the "new" wifi point anywhere in the house. That will be much better than a repeater which will drop the speed. and may work better than another router which may not be positioned for maximum coverage.