What a suitable question to my latest experiences. And COMPARISON!
I recently bought a new computer and decided to take a HD4850 512MB GDDR4 from Powercolor. And I'm more than satisfied with the performance. The most important thing about is that the card is really silent, nearly not noticeable (I spent few Euros to keep the whole machine silent). If you need a silent one with fantastic performance, go for the powercolor. You can flash the card's bios to make it even more silent. If you like to, I send my modified 4850 bios to you, copain.
All I can say is that the powercolor runs pretty fine with Sketchup, which I'm using a lot due to my teaching position.
A friend of mine has the 8800GT in his machine (what a fortuity) and he took his computer for comparisons to me. One thing I must commit, is that the CPUs are different: Mine is an Intel Q6600@ 3.0GHz and his was an Intel dual core something @3.4GHz, so all in one his machine should be faster for SU usage...
So we started with a clean and fresh installation of SU7 and did different everyday stuff. We opened up some small and large models, tried to do this simultaneously, we did selections, orbiting, Animation and jpeg export (what of course not depends on the GPU power) and even did the both available SU-benchmark tests here in the Forum and the result was regarding the different GPUs and CPUs very surprising.
There was none!
The only difference was in the Benchmark tests with a difference of a half frame/s but irregular, sometimes the 4850 was faster 0.5fps, sometimes the 8800GT was. So after everything we tried OUR conclusion was that only the price and the noise makes the difference, which makes the HD4850 512MB GDDR4 from Powercolor to be the first choice.
Even if we tried some games there was, reflecting the benchmark lists on the Internet, no noticeable difference in things of daily usage. Maybe you can measure different values with expensive equipment but for the user himself, every noticed difference must be some kind of voodoo .
-just my 2 cents
