2 - the Geee 92 times two
Alright, let's get cracking. To understand the GeForce 9800 X2 we must first go back to the two GPU cores this product has embedded. Therefore first a quick chat about the graphics processors developed under the name "G92". If you think you recognize that codename, you are onto something. The name G92, is the GPU used in the product you better know as GeForce 8800 GTS, specifically the 512MB models. That product alone comes close, performance wise, to the GeForce 8800 GTX, so imagine what two of them combined can do in terms of performance.
The G92 silicon is based on NVIDIA's newer 0.65nm silicon. Much like the 8800 GT? Is this a respin chip? Rhetorical questions answered "Yes and no, but not exactly". See while this GPU architecture is 95% the same as the good old G80 (GTS/GTX/Ultra) core there are some very distinct differences
The first being the fabrication process. It was moved from 90nm towards 65nm, meaning a smaller die-size, likely resulting in lower core voltages, better energy efficiency and perhaps better clock speeds. The second positive here is that the product has an increased amount of shader processors over the precious GTS 320/640MB series (while being a cheaper chip to make). Let me explain: a 8800 GTX has 128 of these Shader processors embedded onto the graphics core, the old model 8800 GTS (320/640MB) has 96 shader processors, the new 8800 GT has 112 activated Shader processors, yet importantly the G92 is based on the 8800 GTS 512MB which means that both GPUs on the 9800 GX2 have 128 shader processors.
Interesting, because if you take a peek at the clock speeds you'd might even think that one G92 GPU could beat a GTX. Well shader processors are not the only part of the equation, because at the end of the pipeline there this thing called ROPs, and that's where the old GTS (20 ROPs), and this G92 (16 ROPs) become a little castrated compared to the GTX (24 ROPs). There's a new optimization of ROPs' compression algorithm being applied on the GT though, which makes it a bit more efficient.
Next to that, the G92 is utilizing more memory than the GTX, yet the GTX is addressing the memory bus faster (384-bit) as opposed to the G92 with 256-bit, that's less memory bandwidth to play around with.
The performance differential between the two processors is small though, as our benchmarks have shows on the 8800 GTS articles.
A logical question on your end would be, why is the 9800 GX2 a series 9 product, and the 8800 GTS 512MB based on the same G92 graphics core, a Series 8 product?
My answer; I have no idea other then the fact that a Series 9 product sounds better (marketing wise). The G92 based 8800 GTS should probably have been called 9800 GTS or GS as people in the stores right now are really confused.
So in summary, the new features of G92 opposed to the G80, smaller 65nm fabrication process, silicon optimizations (compression algorithms), usage of 256-bit memory and compared to the older GTS models, a high shader processor count at 128 sub-cores.
