I would highly recommend buying Combined Operations, it allows you to play both vanilla ArmA2 and Operation Arrowhead seperately, or together as Combined Operations, and it also gives you access to the "lite" versions of the DLC content (minus missions) for British Armed Forces and PMC for free with lower quality models. The engine for OA/CO has been heavily updated and performance is a lot better in general, also most mods (such as Invasion 1944 *plug*) now require OA/CO as they take advantage of the extra features available.
There's a wealth of community made maps which you can expand the game further, you might want to check out some of the following links to see whats available:
[URL="http://armedassault.info/"]Armed Assault .info - news site[/URL]
[URL="http://www.armaholic.com/"]Armaholic - news site/[/URL]
[URL="http://forums.bistudio.com/forumdisplay.php?f=91"]Official BI Completed Mods forum[/URL]