you could get the EZ IV which is a gba sized slot-2 flash kit solution. it is limited by the fact that it uses a 2gb mini-sd card. micro and an adapter should work, although i've read about it causing some issues. there really aren't any good gba flash kits to recommend anymore, that is a lost aspect of flash kits -- sadly.
there is a gba sized 3in1 kit from ez flash. the 3in1 allows you to flash games to it from a slot-1 which supports it. almost all of the currently available quality slot-1 flash kits will support it, and if they do not then you can use the gba exploader homebrew to flash your games. you will get the highest compatibility with a 3in1 as it runs on actual hardware and not via emulation. the only issue is the battery assisted sram, eventually you will need to swap out the battery and it isn't as easy as plug-and-play.
read more about it here:
http://wiki.gbatemp...._for_EZ-Flash_V
the scds2 will allow you to play gba games via emulation thanks to their built-in cpu. however, it does come at a cost of battery life for your system. if you have the old phat, you may want to consider this as i'm sure your battery doesn't hold that long of a charge anymore. the ismart mm also has gba emulation, it uses the same emulator as the iplayer (both made by supercard but marketed by different companies). the scds2 gba emu is a bit better but all of them will get you gaming.
if you go with the slot-1/3in1 combination i would suggest the r4i gold 3ds by the .cn team (more about it
HERE). its a great cart, well supported, and well built.
you could also consider an ak2i but you will probably experience
contact issues with it.
there are a lot to choose from but the info above should get you started. i recommend that you read the reviews here at gbatemp, found
HERE.
do the research first and then ask some questions. otherwise you are going to be hit up with way to many opinions and not enough facts.
-another world