To be honest, Arma2 and DayZ are poorly optimized, so even with the best PC money can buy you probably couldn't hit 60fps on max settings. Someone correct me if I'm wrong here?
If you assemble your own PC you will have total choice over customization and will be able to build a PC that fits your needs, however, it will take more effort to learn everything you will need to know. Building a PC isn't as hard as you might think, and you'll learn alot from it, it just depends on whether that's something that interests you, and whether you have the time. If you are curious at all I suggest you go look up how much each of the components cost in the PC that you linked to, and compare the total price of the components to the price they are...