I would suggest getting another 2gb of ram for your current PC and a new graphics card. A new HDD too if its needed. Your CPU and Mobo are ok in most gaming situations. Even just keeping your current system and getting a second hand GTX260 or 280 would give you the desired boost. Both would work in your current system as they use PCI-E slots. 500w will cover both.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been