Here is a tutorial on how to upgrade your PSP on 5.50GEN-D3.
-Your PSP must have an M33 or GEN firmware installed already. Only PSPs FAT or PSP 2000 (Slim & Lite) (v3 motherboards are not yet supported)
-At least 64mb free in memory stick
-The battery to be at least 78% charged and the PSP to be on charger.
-This file, it contains everything you need.
What's new in this FW
-Supports all the games in ISO and UMD (up to 12/15/2009)
-Adds the "ISO Speed MS Loading" option (for fast ISO loading)
I tried it with these games:
-Gundam vs Gundam
-Phantasy Star Portable 2
-R-Type Tactics II Operation Bitter Chocolate
Download the file and put the files in the memory stick's root directory. Check if the 550.PBP file is in the root directory before continue.
Now from the PSP's menu, run the updater and select the first option. The file you've downloaded contains everything, so you don't need to do nothing else. When finish, your PSP is ready.
For the GEN menu to appear, turn off your PSP and before turn it on, press and keep the R button and then power it up. You will see the menu.
The best settings are:
UMD Mod : Sony NP9660 -NO UMD-
Use VSHMenu : VSHMenu
Use version.txt : Enabled
CPU Clock XMB : 222
CPU Clock GAME : 222