Here is the cruel irony: The best way to play GTA San Andreas on a "PSP" is to not use a physical PSP at all.

The official GTA San Andreas PSP homebrew is not a standalone ISO. If you find a pre-packaged CSO or ISO file claiming to be San Andreas for PSP, it is either:

Strictly speaking, of GTA: San Andreas for the PSP. The hardware limitations of the PSP—specifically its 333MHz processor and 32MB (or 64MB on later models) of RAM—make a direct port of the massive PS2-era map and assets technically unfeasible.