I have to say yes, because you clearly know what you want. and I have both GTA games and they are very, very cool. Console style games on a portable is really sweet.
Not that the DS is bad, because there are some must haves, but the 3 games you mentioned are well worth the money, trust me.
it does take getting used to the psp as a system and it's worth getting a really, really, really big memory card (like an 8 gb) I have only 1 GB and i need a lot more room. The speakers aren't that hot so earbuds are must, but being able to play mp3s and vidoes and picture is fun. and with the GTA games you can use your own mp3 (after you convert them and put them in the right folder, would have to check joystiq for that) you have the in game radio stations play your music.
I would also suggest MGS portable ops as well.
I have the two GTA story games, and love them, VC is more fun for me, and the radio stations are a blast. I have played the demo of God of War, and it's just like console version, very cool game.
my faves are:
God of War
Activision Hits. some really great Atari games like Frostbit and Stampeed.
Don't sit there and peek around the corner and ask me stupid questions when I'm playing a video game like 'who's winning?' I'M winning. that's all you need to know.