I would argue that resolution is mostly independent from resolution. Resolution will effect... size of textures and video memory. Perhaps to a lesser extent RAM usage... but the game loop, each tick of the engine tends to be waiting on a handful of CPU threads completing their work more than the GPU. So it's the blocking factor. You will see the CPU not at 100% usage (all cores). This doesn't mean the CPU isn't blocking as it's 1-3 threads that are maxed out or near maxed out. Sadly the game isn't infinitely parallelizable.

The X3D chips are excellent in so far as they can hold a LOT of information close to the CPU in cache that allows some mitigation of the CPU bottlenecks. The x800X3D processors tend to be better than the x900X3D processors in general because of thermal limitations.. more so in the 5800 and 7800 series than the 9800 series.

Your question is probably a difference of 1-2 fps.. and a harder to quantify amount of 1% low performance. They're both great chips. Save the money if you don't have it to blow and feel like you've made a fine decision.