Sunray - u may have a 1.5 system now but GTA requires 2.0 firmware to run, as will new PSP games, so that means upgrading to play games then downgrading to run emulators which is gonna be a pain.
I have a PSP and a DS. The PSP is a fantastic piece of kit no question but I prefer the DS for gaming. Its also kind of ironic that the main use I put my PSP to is to play old snes games
Lumines is great yeh, but imo Meteos on the DS is far better and much more suited for portable gaming (its made by the same developers).