Access Point Or Wifi Signal Booster / D-link Wireless N Gigabit Router

Got 1 Linksys WRT54G Router on the ground floor which uses PTCL DSL Internet and 1 D-Link Wireless N Gigabit Router on the first floor which uses World Call Cable Internet of my house. So yeah we have two internet connections.Getting weak signals from both of the routers in my room on the far left side of the house and little to no signals in the basementSo what I want to ask is that should I purchase better routers or should I go with a signal booster of some sorts like an Access Point or a Range Extender?Also is there such a device which would catch signals from both of these differently configured routers ( different internets, different wireless security keys etc ) and then boost them throughout the house if placed in an area where both the router signals are strong?