Ummm your best bet with the NES and GB emus is to get yourself a HDD... If all you want is the emus and games on the drive a 10GB drive would suffice.... you can get a 40GB drive for under $50. Now the atari 2600 emu reads ROMs from the MC.. and it should run the emulator from the USB Drive... same with the TG-16 EMU (PSPCE) reads ROMS from the memory card as well.... so it might run from the USB....
But in both of these cases the ROMS will need to be on the MC
as for that I think that the atari 5200 reads from the MC... so that may work as well.
and if I remember correctly (been a while since I used my AR MAX) you chould be able to browse the USB Drive with the media player on AR MAX and launch the elf files from it.
PS3 Slim 320GB
PS2 v11 exploit w/300GB Maxtor
PS2 v10 exploit w/80GB Samsung - Network Games
PS2 v10 exploit w/60GB Samsung - Network Games
PSP 1001 3.52M33 w/10GB total storage
NDS DSReal 4GB