Actually, 4890 is halfway in between GTX 260 216sp and GTX 275/280 (275=280) "overall".
But when you factor in PhysX, Stereo3D, and CUDA (if you plan on using any of those), then even a GTX 260 192sp could be subjectively better than a 4890.
In terms of speed/power:
GTX 260
HD 4870 512MB...