AGB = Advance Game Boy
AGB-SP = newest model, currently only available in Japan.
I can talk about the AGB-SP now that it is on the market.
I am allowed to say "It is really awesome, buy it."
I think it looks boxy and ugly. My bosses kids had a lack of excitement when they saw it.
But.. It is improved with bigger display, built in backlight, faster proccessor, rechargeable removable Lithium Ion battery, etc...