The only noticeable difference between Unity and Unity2D will be in comparing graphics and how much FPS you can get in each. Since Unity uses 3D effects it will be heavier in graphics and in memory usage. Unity 2D just removes the 3D effects and as a consequence it lowers A LOT the memory usage in comparison with Unity. This is specially true when you have been using the PC for at least 5 days without rebooting/turning it off.
- Better Graphics
- 3D Effects for Eye Candy Users
- More Memory Usage
- Slightly less FPS when playing a game, using a wine game or something that requires graphic power.
- Simple Graphics
- 3D Effects for Eye Candy Users ARE GONE!. Simple and lighter.
- Less Memory Usage
- More FPS when playing games than compared to Unity.
For the CPU I have tested both and I can't really say the differ a lot in CPU usage.
I have tested on a Pentium 4, Core 2 Duo and a Core i7. In all cases the different in CPU was less than 1%. nothing noticeable. But on memory, after 5 days straight you will see the Compiz process in Unity go up to 300mb to 400mb.
So for your answer and taking into consideration the CPU you are mentioning and the power behind it. I recommend Unity 3D since you will never notice a different in CPU. Only in graphics but we are talking between 3 to 10fps less (depending on the game). In my case with an Nvidia 8500, an Nvidia 9500 and an Nvidia 440GT the difference was very very little.
If you are in the border of 30fps for a game, go with unity 2D. If you have a power packed video card that goes way above 30fps then you can safely choose Unity that it will neither suck your CPU or your video performance. The only one that will be less and less overtime assuming you do not reset the PC in several days will be unity 3D with the compiz process.
All in all you can go with Unity.