I can't run DayZ on my machine, the graphics are too intense even when tuned down. It's like a slideshow of watching myself be eaten, which is surely entertaining in an artistic sense.
Running the game on an SSD helps.. a bit. The game is just poorly optimized, it's been that way since ARMA (and maybe OFP)
Borderlands was quite fun (haven't played Borderlands 2, so can't comment on that one), although does run the risk of devolving into multiple-solo-players rather than multi-player, as others have commented.
Hmm, never tried the multiplayer myself, but I would hazard a guess that the Mount & Blade series would benefit people who worked together, even if it's just a pair of sensible people in a team of muppets. But that's pure conjecture on my part - I'm a singleplayer gamer mostly, for the same reasons as some others have pointed out!
If you're going to get Borderlands skip BL 1 and go straight for 2. Borderlands 1 has nothing BL2 doesn't have, and the 'plot' for BL1 is pretty much explained in BL2 so there's no reason to get it for the... 'story'. BL2 story is loads better... simply because it actually kinda has one =p
As for Mount & Blade... get Teamspeak (or a bunch of buddies in steam voice chat, or whatever voice chat program you prefer) and go tear up multiplayer servers. cRPG is a popular multiplayer mod but the community is kinda meh, but as long as you ignore the chat and stick with your butt-buddies-forever you'll have fun =) . You can also get Battalion (Mount & Musket) or even buy Napoleonic Wars and have yourself mini-line battles, or keep tabs on when public line battles take place and attend those.
There is a multiplayer mod where one player plays singleplayer like normal, then when there's a battle he saves the game, hosts a server, and the players who join his server become units in the player's army. I've never tried it myself, but the concept seems cool. I don't even know if it works with Diplomacy mod which is a good SP mod. It was last updated around October 2nd so who knows?
http://forums.taleworlds.com/index.php/topic,150827.0.htmlVikingr is another mod that has some cool features. It's not quite as popular as other ones so the battles are smaller, but that just means each player is that much more important.
Mount & Blade 2 is coming out sometime in the future as well, so who knows what kind of mods or support it will have for this kinda thing?