If you plan on upgrading for Arma 3, I would seriously recommend going with an Intel build and Nvidia graphics card. While it's more expensive, the difference between a Phenom II and Sandybridge or IvyBridge chip is day and night. You probably saw my current specification before I updated my signature, in case you didn't I'm running a Phenom II X6 1090t with two AMD Radeon HD 6870s in CrossfireX. AMD (Formerly ATI) drivers have a bad reputation, especially with CrossfireX. If I run with both cards enabled, I get around ten frames per second less than I do with a single card in Arma 2. Their processors are more suited towards light gaming and general use.
It all comes down to what you're going to use your computer for. Just remember that the minimum/recommended system specifications for Arma 3 will be somewhat obsolete when people start building heavy missions.
Just make sure you have sufficient cooling, if you plan on using the above processor for Arma 3 you may need to overclock quite a bit to get decent performance.