The 65nm 9800GT is literally identical to the 8800GT. There is also a 55nm version of the 9800GT but it is also identical in every way except that it:
Supports HybridPower (meh)
Possibly overclocks a bit better*
Maybe consumes a bit less power*
Maybe runs a little cooler*
*This is my own baseless speculation, I haven't heard any reports of this actually being the case
Now the HD4850 is generally faster than the 9800GTX but this depends a lot on the game and level of AA that you use, hence my claim that they are about equal. Both are certainly faster than the 9800GT. The 4850 also has DX10.1 support which may be relevant at some point.