After watching the trailer from planetside, it looks to me that the game is not that harsh on the system. I'm going to assume this will be the minimum requirements:
CPU: 2 core 2.0 Ghz CPU or higher (maybe even a single core CPU)
Graphics: Graphics card with 512mb of RAM and DirectX 9 support
RAM: 2GB of RAM
NOTICE: Make sure your case supports mATX motherboards and that your HDD is SATA connector not PATA. Also make sure your PSU has 2 open 4 pin power connectors, or a few molex power connectors open to use.
Explanation: This is a great system for the price. The i3 2100, despite being a 2 core CPU can out perform some AMD 4 core CPUs. It has the ability to hyperthread, meaning it can run 4 threads at once, so it simulates being a 4 core CPU. The Radeon 6870 will be able to handle anything you throw at it and it's decently priced. The ASRock H61M-VS motherboard is a great budget motherboard. The 8GB of RAM is overkill even on higher budget gaming computers, but it's always good to have more RAM, and it's not all that expensive anyways.