I remember back in the day doing a lot of tests with WinUAE on various laptops when I was an engineer for the Laptop Shop UK.
Anyway the upshot was the slowest notebook I could get to run OCS/ECS stuff with full sound without a glitch was about 600-650mhz Pentium III mobile.
The PSP has a max speed of 333mhz, this is half the speed and the CPU may be less efficient than a P3 for general code execution, not sure.
However the PSP Go! apparently has a 480mhz capable CPU, this is a huge increase over the regular PSP and no doubt newer PSPs will use this newer more overclockable CPU too.
But if you really want a super portable machine for using with UAE just source yourself a lovely Centrino based Toshiba Libretto. With a 1ghz CPU and a form factor the size of a DVD case you can't really complain about portability.