To be less "critical" of your system than previous responses:
RAM: 8 GB is OK for most games. 16 GB is the optimum amount. RAM speeds don't make a huge difference in games, but you should get at least 1600 Mhz if you wish for good performance universally.
GPU: The 1080 can run most games at high settings and resolutions.
CPU: The Q9550 is very outdated. As everyone else said, you will face serious CPU bottlenecks in certain games. There is a way around this though. The higher you crank up the settings and resolution, more of the load is transferred to the GPU. For example, you would see more CPU usage at low settings at 720p than at high settings at 1080p. However, some games are just terribly optimized and eat the CPU. So it will differ from game to game.
The best CPU for your socket (LGA 775) is the Intel Core2 Extreme X9770 @ 3.20GHz. I should note that just as everyone else has said, it is not worth putting more money into this current system. You should buy a desktop with a newer socket processor, as your system is aged, and will begin to have compatibility issues with drivers and such, if it hasn't already.