The Nachos Supreme is just a smaller version of Nachos Bellgrande. Nachos Supreme contains 440 calories while Nachos Bellgrande has 760.
Nachos Supreme and Nachos BellGrande have the same description on their site. Both have the following ingredients: seasoned beef, nacho cheese sauce, refried beans, diced tomatoes and reduced fat sour cream.
With the Nachos BellGrande, you're just basically getting a step-up on the classic Nachos Supreme which does the trick when you don't want to pay much.
To find your local Taco Bell, use their branch locator tool.