SHORT STORY: The 9800GTX+ would offer you a significant performance improvement over your 7900GTO SLI setup. This performance improvement would be greatest in shader-intensive games, with which the G71-series of cards had poor performance due to its traditional pipeline architecture. In older games, this improvement may be unnoticeable but a 7900GTO SLI setup should be able to handle older games with minimal problems anyway so the point would be moot.
LONG STORY:
Blast to the past! For your reference:
- A 7900GTO is a 7900GTX with slower memory (1320MHz versus 1600MHz). Performance is slightly slower than the 7900GTX.
- A 9800GTX+ is slightly faster than a 9800GTX, which itself is roughly equivalent to an 8800GTX (faster core but slower and less memory).
- An 8800GTX is roughly equivalent to two X1950 XTX's in Crossfire.
In older games, an X1950 XTX is roughly equivalent to a 7900GTX at worst and somewhat faster than a 7900GTX at best. Thus, in older games, you could expect a 9800GTX to give you performance moderately superior to two 7900GTO's in SLI at worst.
But screw all that. It's no longer 2005 and this is a new era with shaders and high resolutions. A 7900GTO is capable of powering most games from 2005 at maximum settings, 4x AA/16x AF, 1600x1200. I would know; I had a 512MB 7900GT overclocked to 7900GTX settings myself a few years ago. But I ditched it for a supposedly-slower X1900 XT 512MB shortly afterwards. Why? The new wave of shader-intensive games was really putting the hurt on G71, which only had 24 dedicated pixel shaders. R580 had 48 dedicated pixel shaders. Even though it was a little slower in older games which needed texturing units, the X1900 XT 512MB was around 30% faster than the 7900GTX in "Oblivion" outdoors while the X1950 Pro had roughly the same performance. That was a taste of things to come. Then "Need for Speed: Carbon" was released.
"Need for Speed: Carbon" used pixel shaders for motion blur and other neat effects. With all the settings maxed out, the X1950 Pro was about 33% faster than the 7950GT, which itself was about as fast as the 7900GTO and almost as fast as the 7900GTX. The X1950 Pro was about half the price of the 7900GTX but had 36 dedicated pixel shaders. At that point I sold my G71 card and bought an X1900 XT.
When the 8800GTX was released, it had roughly twice the performance of the X1950 XTX and everybody forgot about the 7900GTX. There was considerable attention focused on the card known as the 8600GTS, however, which only had 32 unified shaders (pixel AND vertex shaders). At worst, it was a slow 16-pipeline card and performed as such in older games. But in newer, shader-intensive games, the 8600GTS could match the performance of the 7900GTX. And both were eclipsed by the X1950 Pro.
My laptop has a 1.4GHz Core 2 Duo and uses an 8600M GT 256MB. It has a slower core than the 8600GTS and much slower memory - 800MHz DDR2. That is sad indeed. I have a friend with a Dell XPS with a GeForce Go 7950GTX 512MB card. When I played Unreal Tournament III on his computer, I found that my laptop was faster. We both played at 1024x768 but I had more quality settings enabled. (Yes, he played at 1024x768 on a 17" widescreen LCD. No, he did not use fixed-aspect-ratio scaling. It looked about as bad as you would imagine.)
Another friend had a Dell E1705 with a GeForce Go 7900GS before it overheated and burned out on him. He had trouble running Rainbow Six: Vegas 2 at lower resolutions and minimum settings whereas I had no problem running at maximum setting (no AA) and 1280x800 on my laptop. Rainbow Six: Vegas 2 uses the Unreal 3 engine but is not a graphically-intensive game by modern standards.
Thus, it's my postulate that a 9800GTX+ would be a major improvement over 2 7900GTO's in SLI. I don't know about games that, say, use the Source engine, but there are plenty of Unreal-3-based offerings as well as CryEngine2 and Dunia. I did blunder on for several paragraphs and this post is full of unnecessary anecdotes but, hey, good luck with your decision! Now what were we talking about again?
P.S. You may be able to sell your 7900GTO's for $50 each and then pick up a used 9800GTX or 9800GTX+ for $120 or so. That's not a bad investment.
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