The 9800GX2 provides strong performance in at nearly all resolutions (even with my nForce 590 PCIe 1.0 motherboard). The 9800GX2 is a very capable card, especially at the 4M pixel mark. It allows for easily playable frame rates at the highest of resolutions, and at the highest quality settings. I feel comfortable assuming that if you are considering this kind of investment, you've already made an investment in either a large monitor or a Surround Gaming setup. The 9800GX2 isn't the top performing setup in NVIDIA's product line. As of writing this, the crown goes to a pair of 9800GTX's in SLI.
The landscape is a little muddier than it was in the days of the 7950GX2, due to the realization of Tri-SLI. You can use two 9800GX2s for Quad SLI, but Tri-SLI allows you to chain together three cards. This adds a whole other dimension to the cost/benefit analysis. SLI always allowed you to increment you way into better performance. Spend big money on the first card now and then grab the 2nd as prices drop. With Tri-SLI, you can now do the same in three steps, making the last addition very cost effective.
Current thoughts seem to be that Tri-SLI and Quad SLI are overkill right now, at least for the price. There are some upcoming developments from NVIDIA that may make use of the left over processing in Tri and Quad SLI. NVIDIA bought Aegia and their PhysX technology, and they are adding PhysX processing to their 8-series and 9-series cards though a software update. If there is still headroom in Tri-SLI or Quad SLI, then PhysX might be the key to filling it. This idea makes a lot of assumptions: 1) the PhysX implementation comes out in a timely manner, 2) the PhysX implementation works effectively on the GPU, 3) the PhysX processing is able to capture "excess" processing cycles without impacting GFX performance, 4) the games you play support PhysX, and 5) you personally care about physics processing in general. These are a lot of assumptions, and only time will tell how it all settles out. But, this does set up the potential for greater value from an existing or potential purchase.
The 9800GX2 also provides a good upgrade path for someone with an older machine that wouldn't be capable of Tri-SLI, or you prefer a motherboard chipset that doesn't support "traditional" SLI. Grab a 9800GX2 now to get SLI performance, and then pick up another 9800GX2 at a later date (if Quad SLI is no longer "overkill"). No doubt about it that NVIDIA has a strong (if not somewhat confusing) product line, and the 9800GX2 is a serious improvement over the 7950GX2 - in terms of both performance (as expected) and stability. If you have reason to need SLI in one card, the 9800GX2 is a solid performer.
If I had one real "hit" on the card, I would have liked to see the GX2 be a 1.5GB or 2GB card. Getting 768MB on each GPU would help performance in games with large amounts of textures. The single cards moved up in memory, and I wish the GX2 would have followed suit. Having SLI with only 512MB of addressable memory is let-down, and having 512MB in Quad SLI is almost a crime. Adding more memory could have made Quad SLI the top performing setup in the NVIDIA product family.
I plan to continue benchmarking new games on the 9800GX2. I will add new scores here, and to the article on Widescreen and Surround performance. I've finally been able to get Vista loaded. I had to make some minor configuration changes to my rig (including going from 4GB to 2GB of RAM). I did all of this after this review, so I'll have to start over in some DX9 vs. DX10 benchmarks.