Guide to upgrade my Supercard Two (DSTWO 2 in 1)?

[Valzi]

I have a DS lite and want to play DS, GBA, GB, and GBC roms using the DSTWO. What plugins, firmware, etc is best?

It's already working (I used it years ago), but it doesn't have a GBA emulator yet.

I also have an EZ-Flash IV for my old GBA SP. Maybe I should just put it in my DS lite for GBA roms?

 

[Nikokaro]

I also have an EZ-Flash IV for my old GBA SP. Maybe I should just put it in my DS lite for GBA roms?

Sure. To play GBA games is the best choice: a slot2 flashcart.
On real hardware, 100% compatibility (or almost), no slowdowns or graphics and sound issues. This is also my choice.