What are the rest of the current/planned specs ?
IMO Rome2 is a CPU bound game equally , so get a higher end CPU like the i7-4770K , or if possible the 4930K. That will solve the CPU bottleneck. As for the GPU , the GTX 780 HoF will be my pick. Its good enough for Extreme Level settings. That GPU competes the GTX Titan ( which still sells on the 100$ tag ) Also as of RAM , get 8GB-12GB of RAM. Rest og the stuff can be anything as long as its compatible.
If you could provide a budget , I would provide you a rig for that money. Also mention accessories if any needed , as long as the're in the budget.