@fjavi:
That 8800Gt should perform better than my 9800GTX, the graphics that has disappointed me the most. I changed it 4 months ago for another one, because I never thought it would perform better than a GTS. I used to play NFS MW with a GTS and it was much faster despite having less performance. The only game where you could say the GTX performed better was in cod4.
Of course, compared to when the GTX came out, if we compare years later, neither is worth anything. If it's for PhysX, in current games, both lag.
Besides, the GTS OC went from 500 MHz to 700 or more and the GTX 675 to 800 already blown from OC. The only thing is that the 9800GTX was not as expensive as a 680, not even as a 670.
regards
Another game that I really pushed it, precisely, NFS MW (the first one), in the times of my previous 8800GT (not the one I mentioned that I looked at recently), and I tell you that I was doing great, I finished the game like a champion, without problems, and going at about 300 Km/h in urban areas, even if it's through bypasses, had its complication and demand for reflexes.
The OC of the 8800GTS, the one I'm telling you about, is quite low, nothing that a basic 8800GT wouldn't do. But well, one always tends to remember what came out expensive and rendered very well at the time as better than what it was in the end. I already told you that I took both graphics and adjusted qualities in games like Batman AC and similar, and with the same quality, the 8800GT gave more fps and went better (because one thing usually implies the other although there is not at all a "better delivery" of frames, simply because more fps delivered in the same "irregular" way is closer to covering any anomaly than the same with fewer fps).
In PhysX, by the way, I recommend you take a look at this:
http://wwwendigo.blogspot.com.es/2012/04/merece-la-pena-reciclar-una-tarjeta.html
As you can see in that article, the GTX 560 Ti performed better on its own in most tests than using the 8800 GTS 640 MB, but when I used the 8800GT for the same thing a while later... I was pleasantly surprised that it went much better.
It's not that it has more shader power for PhysX (+33%), it's that the improvement goes much further thanks to the slightly more modern and capable architecture in GPGPU/PhysX. The version of CUDA that this graphics card uses is superior, supporting "atom ops", very useful for properly accelerating gpgpu.
So, both in games and as an auxiliary accelerator for PhysX, the 8800GT performed much better now than the 8800GTS 640. The performance of the 8800GT as superior is incontestable:
http://wwwendigo.blogspot.com.es/2012/06/sapphire-hd4850-refurbished-compra.html
In all that analysis, the difference was there again and again. I had an 8800GTS 320MB at the time (and I tried the 640 MB one when it came out, by the way, 400€ at the time, and the 8800GTX... 600€), after a while I switched to an 8800GT for several reasons, and the performance was more solid then, without a doubt. In no case did I miss anything from the 8800GTS.
If years later, when I tried again 8800GT vs 8800GTS 640, I came to the same conclusions and with modern games, it's that the 8800GT won even in the acid test, the one of aging better.
Things are not what they seem, and one has to leave prejudices aside before talking about "instabilities" so cheerfully. Moreover, there are ways to quantify these types of irregularities and I have looked for them and I have not found them.
The problem of Ismarub must be on another side, some kind of incompatibility or instability that we will see why it's there, but it's not directly because of the muscle of the GTX 660, which I have seen adequate (with its level of performance, you shouldn't ask for the impossible).