With its core speed advantage, the HD 4770 is VERY close to the HD 4850.
Shader power is ~equivalent ( .96 TFLOPs vs. 1TFLOP)
ROP power is stronger for the HD 4770 by 20% (assuming perfect scaling with clockspeed)
Bandwidth is ~20% lower for the 4770, but I expect the GDDR5 memory to overclock another 100Mhz or so, granting a 12%+ increase in bandwidth.
Texturing power is also very close, (32 TMUs @ 750 = 24k; 40 TMUs @ 625 = 25k, difference is ~4%, same as shader power)
So the only thing the HD 4850 has a significant advantage in is bandwidth and the fact that it will probably overclock better (can't 4850's hit 700Mhz+?), but since we don't have a concrete 40nm card, it's difficult to determine how far the 4770 will OC.
Overall, it seems a very close race, as the initial Guru 3D benchmark showed, and I _think_ the HD 4770 will make up ground on the HD 4850 once overclocked (mainly due to extra bandwidth).