*UPDATE* Here are the first official preliminary testing results of the 9800GX2 against the 8800Ultra and 8800GTX:
http://www.hardware-aktuell.com/viewtest.php?article=42&seite=3

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So here we have it, the complete detailed specs of the much anticipated GeForce 9800GX2 just hit Expreview.com this morning:
http://en.expreview.com/?p=261
From Fudzilla:
If these numbers are indeed correct, then this card might not be as fast as expected. The GPUs will be clocked at 600MHz, that's 50MHz slower than the 8800 GTS, or the same as the 8800 GT. The Shaders will apparently be clocked at 1,500MHz, again slower than the 8800 GTS which has its Shaders clocked to 1,625MHz.
The memory clock is somewhat higher at 1GHz or 2GHz effectivley for the GDDR3 chips, but this is a mere 60MHz faster than a stock clocked 8800 GTS and is unlikely to make a huge difference.
Not exactly spectacular numbers, but the cards are expected to run quite hot as it is, especially with the two GPU's facing each other, so the lower clock speeds might have been a safety precaution on Nvidia's behalf to make sure they don't end up with a lot of returns.
Here are the specs in comparison to my overclocked eVGA 8800Ultra:
9800GX2
8800Ultra
Specification: 9800GX2 8800UltraStream Processors: 256 (128 per GPU) 128
Core Clock (MHz:) 600MHz 612MHz
Shader Clock (MHz:) 1500MHz 1500MHz
Memory Clock (MHz:) 1000MHz 1080MHz
Memory Amount: 1GB (512MB per GPU) 768MB
Memory Interface: 512-bit (256-bit per GPU) 384-bit
Memory Bandwidth: 128GB/sec 108GB/sec
Texture fill rate: 76.8 GTexels/sec 44.4 GTexels/sec
From what I've heard and researched, I can hope to expect a 30% performance increase going to the 9800GX2. Wow, this means going from 18fps to a whopping 24fps (sarcasm) in GPU intensive Crysis environments. Lmao!