Trying to get homebrew running on your PSP? Want to add a screen light to your Game Boy? Trying to figure out how to work your GP2X? By popular demand, discuss it in here! (This forum is for pre-built handhelds, NOT custom made portables!)
I don't know much about PSP hacking, but I do know one thing. If you're planning to do custom things, never upgrade to an official firmware. (Unless explicitly told to do so)
Playing Castlevania should be safe, but I can't swear on it.
Since you're already on 3.51, you should just go ahead and upgrade to whatever firmware you need to play Castlevania. Any official firmware past 3.50 requires a Pandora battery and memory stick to downgrade. Without knowing someone that has a homebrew compatible PSP, you're kind of boned.