You likely want the XEX format. If you download an ISO, use tools like Xbox 360 ISO Extract to unpack it into an XEX folder structure.
This version is an enhanced port of FlatOut 2 , featuring improved physics, high-definition graphics, and more destructible environments. FlatOut: Ultimate Carnage ISO & XEX - Xbox 360 ...
Sites like Romsfun and Romspure typically host the standard ISO files used for burning to discs or emulation. File Formats & Tools You likely want the XEX format
Use ISO2GOD to convert a standard ISO into a format that can be played directly from the Xbox 360 dashboard without a disc. Game Highlights featuring improved physics