OK, IÂ’ll give you the skinny on this.
I have a GeForce 6800NU and it does pretty well. The result of it only having 128MB of VRAM is really my only gripe about it. Because it doesn't have 256MB of RAM, you can't jack up Anisotropic Filtering to the max, because the more you do that the more texture memory you use up. ItÂ’s the same story with Anti-Aliasing.
As far as the 6800NU being flashed to a 6800GT....
The 6800NU AGP is really a 6800 Ultra with slower core clock speeds, 128MB of DDR RAM instead of 256MB of DDR3, its clocked at 700MHz instead of 1.something GHz (and no matter what ramsinks you use, you won't get the RAM frequencies that high because DDR1 is really, REALLY hot) and it has 4 locked pipelines and 1 locked vertex unit. Its cheaper to manufacture 1 type of product and branch off that, than to make 5 different types of products.
With RivaTuner, you can easily unlock those 4 pipelines and the extra vertex unit, but the chances of it being successful w/o artifacts is slim. And unless you OC to the max before you unlock the pipes and units, your fps increase might be 10fps.
In the long run, itÂ’s not worth it.
Now as far SM 3.0, itÂ’s your call. Valve announced that the ATI X-series cards with SM 2.0b will be able to run it on high with HDR like the Geforce 6-series. In Splinter Cell: Chaos Theory, the Radeon X800Â’s can run the game maxed out like the GeForce 6-series thanks to a recent patch. Far Cry has HDR, but will only work on SM 3.0 cards. BF2 has support for SM 3.0 that increases the shadow quality, but then again, the shadows look like trash, so they would be off anyway.
The X800 Pro, because of its higher clock speeds, and more memory, performs better than the 6800NU in most everything except OpenGL titles. My 6800 has trouble with HL2, but I think that may have been a motherboard issue, I havenÂ’t tried it with my new motherboard, but IÂ’ll see what happens later. My 6800NU can run any game (with the Exception of Fear and BF2) on the highest. Far Cry at 1280x1024 maxed with 2xAA and 8xAF is truly a religious experience. Doom 3 at 1600x1200 is immaculate, and Splinter Cell: Chaos Theory with HDR, Soft Shadows, and Parallax mapping is Immortal.
So, if you wan better framerates today, get the X800 Pro, but if you want more flexibility tomorrow, get a 256MB 6800NU or GT (depending on the price)