Welcome to Armaholic and the series.
A few topics below you can find this:
- Anyone still playing ArmA1?
I doubt there is much activety left, most people have moved over to Arma 2and Arma 2: Operation Arrowhead.
Specially if you want to jump into the MP aspect of the game it is very much adviced to move over as well.
If you are limitted by your PC you can of course still enjoy Arma very much but mostly the singleplayer aspect. There are hundreds of addons and missions to keep you busy for some time.