Far Cry 1 is cool at first but be prepared that there's a bullshit difficulty spike about a third of the way into the campaign. It's possible to get used to it but I know many people who got turned off the game completely because of it. It's also a pretty linear game compared to the sequels, and it's a completely different tone and style of gameplay. It's more similar to Crysis than Far Cry sequels, which shouldn't be a surprise considering that it was the first game made by Crytek.
Far Cry 2 is the Far Cry you know, although it's also pretty different from later games in the series. It's a much more immersive experience - the HUD is minimalistic, the map screen is a literal map that your character pulls out and holds in front of his face, guns get jammed and you have to replace them if they get too dirty, you have to take malaria medication regularly, your companions can get permanently killed, and the overall tone of the game is very gritty and serious. I love it, personally, but those immersive elements is what many people disliked about the game because it served more as a distraction from shooting guys than something that enhanced the experience. It depends on the person, really. Oh, and also there are the patrols that constantly respawn and once they spot you, they will never ever stop chasing you until either you or them are dead. This can be annoying but manageable.
Oh, and please, for the love of Christ - DO NOT PLAY THE PS3 VERSION. The frame rate is horrendous and aiming controls suck balls. You're seriously better off grabbing a PC version which is by far the best way to play Far Cry 2.