Don't expect a big upgrade, your CPU will bottleneck newer mid-range / high-end GPUs big time. Planetside 2 does require a decent CPU.
Radeon 7750 requires Little power and does a great job. You won't get highest details, but you will see a major difference - If your CPU allows it.
You might consider getting a new desktop if your budget allows it, if you'd like to keep up with new and upcoming games.