LOL. Welcome to overclockers forums.
I would not listen to those who say 'should have bought nvidia'. The only problem i have had in a long time is that Battlefield 3 sometimes does not start first time but that is a problem with the game rather than my AMD graphics card. Plenty of people have trouble with nvidia and there drivers. You are better finding out the problem and fixing it rather than swapping graphics cards as all you will probably get is a new set of problems.
I completely agree. You'll get people who will be having problems with their computer and some one like AWPC or Arknor will come along and go "herpity derp, should have gone nVidia, you never get problems with them", person listens to them and goes out to buy an nVidia graphics card. For the people whose problems go away, they end up becoming jaded and think nVidia can do no wrong but everyone involved there completely ignores the fact that a major part of the system has changed, and the same thing often happens with people having problems with nVidia graphics cards, they get an AMD one and the problem goes away. It doesn't mean it's the fault of the brand of graphics card, there's far more to computers than that and people who utter things like that, you shouldn't listen to at all because they don't really want to help you they just basically want to indoctrinate you into their idiotic little belief system of how nVidia never has any problems, ever. The same does happen with AMD, but for some reason there's far more people who do it over nVidia than AMD *shrug*.
I would just re-install windows - install motherboard - graphics - sound card - network drivers (make sure that ALL these are all the latest version from the manufacturers website). Then, update windows and install ant-virus/firewall. Re-install your programs and games (make sure your games are updated with latest patches). Sometimes certain components of a PC will not like working with other components so they can fix issues by releaseing updated drivers for sound cards and other components.
This will be the main issue really, it doesn't seem like the PC has been set up properly.
Your games should run fine on medium settings with a 6870
I reckon he'll get more than medium out of most games.