The game that started it all: Operation: Flashpoint -- now known as Arma: Cold War Assault thanks to a trademark dispute against Codemasters, who took the name for their new offering Dragoon Rising, made by a completely different development team. If you want the game made by the original developers, this is your ticket. Make sure you know for sure that you aren't getting anything truly new with this: just the original (and arguably the best) military simulator ever to hit the store shelves. (Psst, it's also cheaper here by $1 than from a certain vaporised water service, which is charging $5.99.)
Anyone with a halfway modern gaming rig will be capable of running this smoothly and flawlessly. However, keep in mind that the system requirements posted by Gamersgate are the absolute minimum, as stated for the original release. Like most of the other games in the ARMAverse, the minimum requirements are truly minimal -- the ability to run the game at absolutely minimum draw distance, detail, and playability. You'll want to at least meet the recommended requirements as stated by the developer (which should be very easy if you've upgraded your computer any time since 2004):
Pentium III 1GHz CPU
16 bit DirectX compatible Soundcard
DirectX 8.0 compatible 3D graphics card with 32Mb RAM
800Mb Hard Drive space (after installation)
Back when the most advanced military realism was available in games like Rainbow Six, this game blew them all out of the water when it decided to approach combat on the modern battlefield instead of just close quarters battle. This of course required an entirely new approach to battlefield rendering. While the various games that succeeded it improved the polygon counts and view distance dramatically -- both within the ARMA series as well as within completely different technology offerings like Far Cry and Crysis -- this game was still groundbreaking and revolutionary when it first came out.
And even though this is technically just a re-release, it's still groundbreaking and revolutionary in its own special way. By buying this you own a piece of military simulation history. Many people prefer this version to that of ArmA: Armed Assault, which was largely similar but also felt by many to be "largely similar" -- nothing new. Well, while this version is also still Operation: Flashpoint, it includes the original Cold War Crisis plus the Resistance campaign all for a tidy $5. Again, because of the aforementioned conflicts with Codemasters, the Red Hammer expansion is curiously absent, but don't let that stop you from picking up the game that launched the series.
Just don't complain about the three-round burst on the AKM. ;-)
In all fairness, it's worthwhile to mention the Cold War Rearmed mods. They're nowhere near finished yet and based on the going rate they probably won't be done before ARMA 3 comes out... but if they ever get done, you'll get the best of both worlds -- the Cold War Crisis campaign in a fully modern engine. You don't even need Cold War Assault in that case, but I figure it's worth a buy anyway. On sale, if $5 hurts your sensibilities. Now, otherwise.