Welcome to Armaholic and the game
Arma 2 and Arma 2: OA should work with your system. Maybe not the best performance but you should be able to play the game, surely in SP.
Make sure you set all settings to low to start with and see if that fixes that issue.
Now, as for getting the hang of this game...............take your time and spend time.
This is not a game you boot up, and know how to play in a few hours. This takes a lot of time and effort.
Play the SP campaign, play some scenarios, read the manual so you known the keyboard layout and functions etc.
Also playing MP is a good idea however I do not
recommend joining public servers as your new you wont like it there.
Check the advertising section to see who is looking for new members and does not mind having to teach and show a new player how it all works. Most clans/units/communities greet new players with a lot of respect and are willing to teach if your willing to invest time and effort