In case you haven't already gotten both of them:
I downloaded Far Cry 2 (shhh) after I built my new PC (I wanted to see what the 4850x2 could do) and I am IMMENSELY glad that I didn't actually waste the money to buy it.
To begin, its obvious that the graphics are good, that I'll concede in a heartbeat. However, this game is, by far, one of the most repetitive games I've ever played, ever. The mechanics are fine, yet everything is presented to you in an incredible linear format. The only times assigned missions attempt to break from "travel from A to B to kill/rescue C" is when you sometimes get cell phone calls from allies, offering a different way to complete the mission. The only issue is that the missions are so single-minded that variation is non-existent. What I mean is this: you say that you've played Fallout 3, so you'll know that there are multiple ways of completing objectives, but moreover, there are multiple ways of playing sides off of one another, completing only parts of objectives, screwing over original parties, etc. Take for instance the things you could do for the Tenpenny Tower missions; whether you want to aid the ghouls or Tenpenny is just the basic part, but you could actually accomplish some tasks for Tenpenny, get payed, accomplish some tasks for the ghoul leader, get payed, and then screw over one, or both, for an even larger sum of money. Throw in the Alistair Crowley side missions and its even larger in scope.
Now I'm not saying that I expect Far Cry 2 to be nearly that open for player interaction, but I do expect some flexibility outside of the obvious "go in with an assault rifle, or sniper rifle, or rocket launcher, or car, etc" formula that they pawned off as being open-ended. Instead, I was treated to missions where I'd try to go along with my ally's plan, and then complete it a different way so I could potentially be payed by both sides. Instead, what I get is a phone call from my ally saying "well, I guess you didn't like my plan" and his objectives being cut off. How did he magically know that I didn't do his plan? Its not like he was on the mission with me; the game just recognized that I deviated from a surprisingly strict path and just miraculously ended the ally's cooperation.
Outside of this are incredibly obvious AI limitations. First of all, there's really no such thing as stealth in this game, even at night. The game programmers went the route of Rainbow Six: Vegas (on the co-op maps) and made the enemies omniscient in their awareness of you. Sure there were times when I snuck up on people, but then the game rewarded me with a death scream by the victim when I used the machete, thereby alerting everyone around him. In short, you're definitely not Sam Fisher in this game. Moreover, EVERY mission you get from the two factions fighting against one another (on both maps...so that gets really refreshing) is stated to you as a "secret" mission. The leader always assigns the mission, and tells you that no one in his faction knows that you're helping them out. So what does that mean? Oh right, that not only do have to drive out to fight the enemy, but on your way any supposed "ally" faction guard posts or patrolling jeeps attack you on sight. Let's put it simply: this wasn't a plot device to do "secret" missions for factions, this was an afterthought after the game developers realized that they had placed a faulty faction system into the game. The very first Mercenaries was able to construct a believable faction system, yet this game fails miserably at it.
Back on the subject of guard posts. They're everywhere. Everywhere. And what a joy it was to find out that even if you killed everyone in the guard post, they would just respawn about an (in-game) hour later. So basically I was forced to spend more time attempting to circumvent guard posts in order to reach my mission than actually completing the mission itself. Maybe you're asking, "why didn't you just drive directly though the posts"? Because in Far Cry 2, vehicles are evidently made out of glue and balsa wood, and in only a few shots will start smoking, if not outright explode.
I have a lot of other qualms, but I've already been diatribing long enough. The short of it is this: don't buy it, its not worth it, regardless of graphics. If you want to play it, go piratebay or wherever.