Any Grand Theft Auto game is your ticket to hours of playing the PS2!
If you can wait for San Andreas, do what one guy did in the GTA Forum, and go from GTA 3, then to Vice City, and now on to San Andreas. It gives you the whole deal on what the hype has been all about, I guess.
Also, buy GTA on the PS2 if you can play it well. I personally prefer GTA on PS2, but that's because I'm more used to the controls, I love the sensitivity of the buttons (how you can half-accelerate, walk instead of run, etc), and I'm too lazy to be bothered getting a Controller for the PC. The PC version only looks better, and you can mod it, but mods do have a tedency to crash your game. I don't care too much for mods anymore, anyhow.
For other games, Mafia is good. I played a fair bit of that, although I was too lazy to complete the last mission, even though it was fairly easy. Don't buy True Crimes: Streets of LA. Don't get trapped by it. Same goes with Driv3r! Those games are awful!
Up to you on those though, they're just my opinions.
I never got around to playing MGS, but it sounds like a definate must-play, even for me. Gran Turismo 3 is fun. Even though I don't own the game anymore, it's still a great game to have in your collection. Ignore all of the boring comments. Just take it this way: if you like racing games, and like to race, improve times, etc, then get it. If you get easily bored in Racing Games like this, then move right along.
As for all of the arguments on GTA:
GTA's graphics are better on the PC. Much better if you have a good PC. It's nothing to get too excited about, but if you're a lover of graphics, then you would like it more on the PC. It's a lot sharper.
The sounds don't mean a thing, as it depends on your speakers. The PC version did NOT enhance the sound quality.
Manual aim is better on the PC, although I prefer Auto-Aim a lot. Some people have trouble with it, but it's really not all that hard to target enemies, especially in San Andreas, and it's really no different to Vice City, regardless of what others may say.
Also, there is slow-down. It's noticeable, but it doesn't happen too often. Seeing that you're getting a new PS2, you won't get as many problems with the game as some.
All-in-all, GTA games are the ultimate replayable games. For any questions about it, head over to the GTA Forum. You're all welcome there.
I'm cheesy, I know.