Counter-Strike is the biggest online game of all time, it's a Terrorist vs Counter-Terrorist mod for Half-Life (so buy Half-Life and download Counter-Strike) it isn't technically strenuous by today's standards but you will need atleast a bog standard 3D card (as with just about any game of the past 4 years :/) to play it.
I'd be up for playing that, I was in one of the best clans back in 99/00 but stopped playing when it became shit, I started a few games last month and it stuck again
You pick your allegiance, then what sort of terrorist or Counter you are (everything from a weird geek looking bomber to the SAS and Navy seals), buy your equipment, Colts, MP5, P90, Grenades, Flash grenades, armour etc then you compete in 1 of 2 kinds of game (1. Terrorists plant bomb and Counters try to defuse it or 2. Counters rescue the hostages while the Terrorists be annoying, it's all good). Rounds last for 5 minutes roughly, so if you die after 10 seconds you have 4 minutes 90 to watch the rest play, if you die after 4 minutes you can rest assured there'll be a few folk watching you try to complete your objective in the last minute :P
The round ends when either 1 team is taken down, when the bomb is defused or when all hostages are rescuedor capped by the alquida.
If that makes anyone moist I'd suggest you buy HL and download this.
Rob