The term "ISO" refers to an exact archive copy of the data from a GameCube optical disc (NOD format). Technical details of the GameCube release include:
Scale up to 2x (720p) or 3x (1080p) to sharpen the car models and eliminate jagged edges. Need For Speed Underground 2 Gamecube Iso Highly
If you want to optimize your retro racing setup, let me know: The term "ISO" refers to an exact archive
format. If you are using the Dolphin emulator, you can actually compress these yourself into a format called Need For Speed Underground 2 Gamecube Iso Highly