Shindolim, Gangnam, and the shincheon (next to Yonsei Uni.) all have Taco Bell restaurants as well.
Also for good Tex Mex I really like "On the Border." They have unlimited chips and salsa with ok prices on individual orders. Not nearly as good or delicious when it comes to big sets, just my opinion though. The three I know of are near Yonsei (again), IFC mall, and one at Samsung station.