This is the most popular choice for the FC3000. It utilizes a version of OpenDingux optimized for the device’s hardware. It transforms the handheld from a "toy" into a legitimate emulation machine.
Locate the CFW image specific to your version (V1 or V2).
Most CFWs allow you to toggle between "Scaled" and "Original" aspect ratios. For GBA, "Original" looks best but will be smaller; "Scaled" fills the screen.