Yes, it would be worth adding one more 4gb stick. Relatively speaking, the memory is what is bottlenecking you. Most AAA games recommend at least 4 GB Ram with COD Ghosts requiring 6 GB to even start. These days 8 gb would be the minimum recommended for gaming especially when your using such a powerful gpu as a r9 290.
Yes, it would be worth adding one more 4gb stick. Relatively speaking, the memory is what is bottlenecking you. Most AAA games recommend at least 4 GB Ram with COD Ghosts requiring 6 GB to even start. These days 8 gb would be the minimum recommended for gaming especially when your using such a powerful gpu as a r9 290.