if you are running super high rez + AA + AF 8800gtx
Super rez and not soo much AA + AF 9800GTX
The 9800's have more power but the limited amount of memory effects it in some situations.
The power use is lower, but its not too bad when you factor in the performance of both cards. about 100 watts for the 9800gtx and about 132 for a 8800gtx.
With the price of the 9800gtx does offer better performance per dollar. Its a shame that nvidia did not give the 9800's the memory the deserve.