It sounds like that computer should run the game very well. It requires a C2D and 7800 GT. You have an excellent GPU, it is definitely being bottlenecked by the CPU but that shouldn't be an issue in that game.
You could probably just upgrade the CPU/motherboard/RAM and be set, but I feel like your computer should have no problems with that game.
For example, this is what you would need to upgrade:
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor ($226.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($33.98 @ SuperBiiz)
Motherboard: Gigabyte GA-Z87X-D3H ATX LGA1150 Motherboard ($129.99 @ Amazon)
Memory: G.Skill Sniper Series 8GB (2 x 4GB) DDR3-2133 Memory ($81.99 @ Newegg)
Total: $465.96
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-01-10 08:07 EST-0500)
If overclocking is not a concern, you could save about $100.
I would make sure your GPU drivers are up to date and make sure the game is patched up.