The problem with Halo is that it needs a very good gfx card to play. I have a GF2 and if i turn everything off/down it still can't run it at playable framerates. I have heard that Desert Combat, a free mod for BF1942, is very, very cool. It is set in modern times with war waging against the middle-east.
I would have to say though that the best game to get for online play would be Tribes2. This game has kept me entertained for over two years and never gets boring. It is only online (unless you play with crappy bots) but has no subscription fee. It is a first-person shooter mostly played with two teams, capture the flag. It is very fast due to everyone having a jet pack. I strongly reccomend you buy or get the demo to this game.