Everyone's rig and specs vary, but my experience is similar, having kept both FSX Gold and P3D v4.3 on the same system. P3D runs smoothly, no OOM's and very rare lockups (often user error). The visuals are stunning, particularly if you run e.g., Orbx FTX Global, et al, Active Sky and Tomatoshade whereas FSX struggles to get frames, to load textures when switching views and to achieve smooth (AAS) lines. All using the same machine. A BIG difference. My understanding of Lockheed's move to 64 bit was that the whole rationale was this would greatly REDUCE the strain running the sim puts on the CPU and other bits. I haven't touched FSX since except for the occasional brief test.
I sense that the reason so many still seem hesitant to move over to P3D is that they misunderstand this and assume, as with the move up from previous versions of FS, it always means you need a more pricey and powerful machine to make worthwhile. Sheesh, P3D costs less than many single add on aircraft!