What are some good online FPS games?

I just got a new comp, kind of low-end as far as gaming goes, but I just wanted to know what are some great shooters for online that you all are playing?

I thought about getting halo 2 for pc because I used to love playing it on xbox live.

My specs are:

AMD Athlon 7850 Duel-Core proccessor 2.8ghz

ATI Radeon 4770

2 gig RAM

Windows 7

i'm interested in new or old games, thanks for your responses.

Does Crysis have a good multiplayer? does anyone have any reccomendations?

my recommendation is the recomendation thread, if the pc forums have one lol. others try counterstrike source, teamfortress2, left4dead or um yea i heard crysis is good. i coudnt play it tho.

Team Fortress 2 (Get the Orange Box), Battlefield 2/2142, Call of Duty 4, Counter Strike Source,

CounterStrike: Source, Team Fortress 2, Call of Duty 4/WaW/Modern Warfare 2 (when it releases), Americas Army, Left 4 Dead or Killing Floor.

I think left 4 dead is just awesome, both vs and co-op. Alot of people like team fortress 2, and for a more classic experience UT3 is alot better than ppl give credit for.

Left 4 Dead is my #1. I also recommend Counter Strike: Source.

Counter Strike is always fun. Crysis has a decent online multiplayer.