Start off with getting a
PSPlus subscription, especially if you also have a PS3 - nothing beats free games on a monthly basis. Next up there's
Virtue's Last Reward if you're a visual novel fan and enjoyed 999 on the DS.
MGS HD Collection is quite a good buy as well, but only if you haven't played those games in their original form and don't have a PS2 - the pack doesn't bring much more new content. Next up there's
Persona 4 Golden, but you already mention that yourself. Following that,
Assassin's Creed: Liberation is quite an awesome-looking PSVita exclusive if you're in any way interested in stabbing people in the back.
There's also
Ragnarok Odyssey if you like the "Monster Hunting" genre at all. For all your "shooting other players in the face" needs, there are
Call of Duty: BlackOps Declassified and
Resistance: Burning Skies - not the best shooters overall, but certainly the best in a handheld format. If you enjoyed Super Smash Bros. Brawl, you should consider getting
PlayStation All-Stars Battle Royale, which pits Sony-related characters in a similar fashion. If you like the standard fighting games though, have a look at
Ultimate Marvel vs. Capcom 3 unless you already have it on another console. For racing games, I heard good things about
Sonic & Sega All-Stars Racing: Transformed which is practically Mario Kart a'la Sega, alternatively
Need for Speed: Most Wanted if you haven't played it on a home console yet or enjoyed it to the point where you'd like to play it on a portable.
I specifically didn't mention any games available on PlayStation Plus, but if you're not getting it,
Mortal Kombat is absolutely stellar,
Gravity Rush/Daze is breathtaking and
Uncharted: Golden Abyss can easily be considered a killer app for the system. There's also
Wipeout 2048, but I'm not sure if it's still in the PSPlus section, I don't have access to the NA PSN Store so I can't tell. In any case, it's Wipeout - you either love it or hate it, I personally enjoyed the demo and plan on getting the full game eventually.
I also
strongly suggest that you get the 32GB memory card - I have one myself and let me tell you, those cards fill up real quick. The average download is between 500MB and 2GB, the largest download right now (AFAIK) is Resistance: Burning Skies, weighing at 3GB. I absolutely hate switching memory cards so I chose the largest capacity Day 1 and I'm not regretting the decision - if I run out of games, I can always fill it with a gajizillion of Demo's as well as free content, of which there is plenty on the PSN Store. The cards are ungodly expensive, yes, but you
only need one, so in the long run, it doesn't hurt your wallet
that much.