I bought it just for DayZ and for the past 44 hours have literally done nothing else, bar sleep for about 8 hours. It's the best thing I've played in years, last game that did this to me was Morrowind >_>
Arma 2 in general is one of those love it or hate it games. There's a free multiplayer demo version that you can try to see if you like the style. I can't comment on the zombie game as I've only watched about 5 minutes of some dudes playing it.
No, you can't play it offline or in locked servers. ARMA II is relatively demanding, but with your specs you'll be fine.
If you like slow-paced gameplay, and don't mind the lack of progression (when you die, it's back to square one), then it can be great fun. I personally enjoy it a lot, but I already had ARMA II; I'm not sure I would have bought it JUST for the mod in the state that it's in.
I bought Arma2 just for DayZ, and I'm completely satisfied. It's still in alpha and sometimes it may be difficult joining servers, they're usually packed on weekends and over working the master server, but it is a great experience.
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I would wait for the mod to develop more or at least for ARMA 2 CO to go on sale somewhere. The mod may be interesting but a lot of people are likely to get frustrated with it between the problems of the ARMA 2 engine and the mod still being an alpha.
I bought it just for the mod and love it, I'm playing the prerelease 1.7 version though since its more stable and framerates are higher (server causes poor framerates) but theres not alot of 1.7 servers just 1.6. The biggest learning curve is how to properly use the inventory its really confusing at first the way they set it up.
Im not gona lie, but i hate one thing about Day Z that completely ruins it all for me: Player Killing.
The longest Ive lasted with a character is about 10 hours. Ive gone through 5 different characters. The cause of death for all? Player killing.
I understand why people love this game. I can totally see why and I would too, but this damn player killing is annoying. Its like, forget teamming up and fighting zombies together. Just go and kill anything that moves other than yourself.
I was ambushed by a group of other players (there were 3 of them). At first they were just like whats up then bam, im dead.
Id get back into it if they would make a server with no player killing. Until then, its whatever for me.
...but still, id recommend you try it regardless. People from all over are loving it. Im probably just having tough luck on my experience with this mod,