First of all , welcome to the Site
Next... You cannot physically increase the RAM but there is a specific software mod which will allow you to use the entire 32MB RAM on ur model...
Besides that for more usability you can add a second analog stick to your psp on the right.
Then you can add a photofast adaptor internally for 32GB of storage..
You can also add a louder 3rd speaker in the UMD bay but you will lose the UMD drive for the previous two mods.
You can also do a USB Charge mod , internal Chotto-Shot Camera mod , i think an internal GPS is also possible.
If you have skills you can even flash a 360 chatpad to work with the PSP.
Looks wise , you can do case mods and sound reactive lights , jewels , etc...