Take sony's old or new 2200 mAh, a Slim, and a Slim extended battery door(black and silver in the new bundles, rest on ebay) and get one amazing powerhouse. The math goes like so:
Original PSP-1000 lasted 3-3.5 hours of gaming on a 1800 mAh. The 2200 would only boost the life to 4.27 hours. Hardly worth the $40 price.
Slim PSP-2000 achieves the same 3-3.5 gaming hours with a 1200 mAh battery. The 2200 in this case yields a whopping 6.5 hours of gaming.
So whats the #1 reason to get a Slim? The ability to DOUBLE the gaming life with an
official, bug free battery.