Any single core 3GHZ CPU, which is usually the best you can get without overclocking as far as I know. I haven't seen any
ArmA doesn't use multiple cores, so any multicore you get should 3 GHZ each so ArmA can still use it, and if you want to use the multiple cores for other games.
You can get a
3 GHZ quad-core for $550 on newegg.com
That is as almost as good I assume as the one I bought but cheaper. Apparently you can overclock it to 4 GHZ too without extensive extra cooling. With this, theres no need to buy the $1500 Intel 3.2 GHZ quad core which I guess is the top of the line.
As for video cards, ArmA doesn't use SLI either, so any fast single video card will do.