Hey,
Ive just upgraded from a 7900GTO (See rig in Sig)
My GTO was clocked quite high, 710/810 so beyond GTX speeds. The rig and the card scored 7000 in 3Dmark 06.
Ive upgraded to a BFG8800GTS 320mb OC2. Now scores 10010 (@ Stock)
My findings @ 1440x900.... (Real World)
BF2142
Can't say ive noticed a lot of difference, Image quality is a lot shaper than the 7900GTO. The 7900GTO used to drop around the high 30s on a 48 player map with loads going on, The 8800 does not seem to dip below 50fps.
Test Drive Unlimited
Image quality here, Big difference. a lot sharper, colours look a lot brighter. Around 10fps quicker on adverage.
CS : Source
Hardly any difference here as expecting. Quite old now. Normally sits around ~200fps. The IQ again seems better.
Lost Planet Demo
Not a fair comparison as Ive only installed the DX10 demo with the 8800. However there is quite a big difference. When outside the 7900GTO sits around ~30fps. The 8800 seems to glide along at 50fps.
Image quality is so much better with this game, sharper and brighter colours.
Noise
Both cards are very quiet. The GTO fan never seemed to spin up. My rig is pretty quiet. The 8800GTS runs a lot hotter, but yet to hear the fan spin up to 100%. No real difference here between the two.
Overall
Overall im pretty happy, the biggest difference is the IQ. Speed wise its not like upgrading from your GF2 to a 6800Ultra, but there is a small speed increase for DX9 games. The gap will probably get bigger once the DX10 games start appearing.
I manage to sell my 7900GTO for £100 to a mate who was very happy. The 7900GTO is still a great card by todays standards.
Hope this helps.