I am playing Minecraft 1.7.5 with Optifine on high settings (all video settings except OpenGL set at fancy). I get a consistently high framerate on average: 90-140 FPS. However, I also experience constant stuttering, and occasionally even massive framerate drops that at their height bring the FPS down to even 20. Also, the VSync does not work.
The PC specs are:
2.6 GHz Dual-Core Pentium;
3.12 GB RAM; Nvidia GT 610 Video Card