Congrats

Hope you have fun with it.
The Halo games are pretty overrated in my oppinion but they are fun to play and and worth picking up. Halo 2 leaves you with a huge cliffhanger ending though so try to not care about the story

It depends on what kind of games you like.
For first person shooters I'd recommend
Chronicles Of Riddick - Escape From Butcher Bay. It's one of the best I've ever played.
Star Wars - Republic Commando is also great. It's like a Rainbow Six in the Star Wars universe but with simplified squad mechanics. Pretty much a straight-forward shooter.
If you haven't played the Max Payne games already on PC or PS2 I'd recommend these, too.
Doom 3 and
Doom 3 - Resurrection of Evil are really good. I'd say if you haven't already played them on PC get them. Same for
Return To Castle Wolfenstein. Last but not least,
Black, it's awesome.
For RPGs the two
Star Wars Knights Of The Old Republic games are among the best.
Deus Ex: Invisible War is a mix of FPS and RPG and pretty good.
Indigo Prophecy and
Oddworld: Strangers Wrath are more like Adventure/Action-Adventure games but I'd recommend them without hesitation.
For Fighting I'd recommend
Soul Calibur II,
Dead Or Alive 3 and if you're more into 2D fighters
King Of Fighters Neowave,
Capcom Vs SNK 2 EO and
Guilty Gear X2 #Reload. The latter is a bit on the hard side though.
The Warriors is great and deserves more praise than it actually got. It's basically a 3D Final Fight but has a great atmosphere sound and story.
Lastly for straight action games there's one to rule them all
Ninja Gaiden Black. It's insanely hard on the higher difficulties but when you beat it you feel like you've accomplished something groundbreaking
Oh, and seeing that you came to like Metal Gear Solid lately, there is
Metal Gear Solid 2: Substance