One can argue that the recent success of Radeon with the 48*0 line is only one battle in a long war that nVidia has largely under control, but even if this is just a blip on the radar, a 4850 now for a couple hundred bucks and another one later (maybe in 8-12 months as prices drop) will be pulling solid frame rates on the latest games for a couple years. Whereas SLI was almost a must have for people demanding multiple GPU performance, nVidia's chipsets will have to stand on their own for the time being.
The 680i is rather old to be considered a newer chipset, particularly as almost all boards based on it can only handle Duo 45nm CPUs (Wolfdale), not the Quad 45nm ones (Yorkfield), but it serves as a good starting point. Despite a difficult launch period that saw users waiting for bios and driver updates several times, as the top end of SLI chipsets, it found its way into most high end gaming systems. Its impressive and easily achieved overclocking feats when Intel was still regarded as rather conservative on this front along with the 680i's long feature list, including 1333MHz FSB support, even earned it a place in the systems of many users that only run one graphics card. It comes in two flavors; SLI and LT. The latter has support for 2 way SLI on x16 slots, the former adds 3 way SLI, though the third card has reduced bandwidth, and support for DDR2 memory speeds up to 1200Mhz. Note that this memory speed requires SLI-ready memory, much like the aforementioned XMT system Intel uses.
The only real advantages Intel based boards could offer before they released the 3 series (e.g. 946PL or P965) were reliability and ease of use; having to upgrade drivers or flash a bios on these boards occurred rarely if ever. Also, Intel's chipsets came with a slightly faster southbridge: the ICH8.
Without getting too technical, the northbridge communicates with the CPU, memory, GPU(s) and the southbridge is the part of the chipset that communicates with things like storage drives (SATA or PATA) and USB ports. The two bridges also send information to one another. Also note that "exotic" multi-GPU configurations (those with three or more cards) will often talk to the third card through a PCI-E slot connected to the southbridge.
So the 680i made big gains on Intel's older chipsets, but how do nVidia's subsequent offerings stand up to the 3 and 4 series chipsets?
The 780i SLI adds another full width x16 slot for your third GPU and brings 32 lanes of PCI-E 2 support by connecting an extra chip called an nForce 200 to the Northbridge. You will note that 3 times 16 equals 48, that is because your third GPU communicates through the southbridge. This is the same southbridge one finds on the 680i, the one that was slower than the ICH8, let alone the ICH9 found on the X38 and X48. Furthermore, the nForce 200 handling the PCI-E 2 connections doesn't actually communicate with the northbridge at the full speed available to 32 lanes of PCI-E 2, possibly forming a bottleneck. Finally, considering that it has the same memory and FSB support as the 680i, it is fair to say that nVidia needed something to run Penryns in general, not just Wolfdales, and out came the 780i.
The 790i SLI moves things along by hooking the 32 PCI-E right to the northbridge where they belong, but still leaves your third card on the southbridge. And guess what? Yup, it's the same southbridge from the 680i and 780i. It does move ahead in other regards though: most notably support for DDR3-1333 memory and 1600 MHz FSB. The 790i SLI in Ultra trim doesn't appear to have much on the standard model, specification wise the only difference is DDR3 support for SLI-ready memory all the way up to 2000 MHz. The Ultra though, happens to be assembled from the highest-testing chips in any particular batch, so it should have better overclocking headroom, though if that is to a noticeable extent it has not been established.
Another issue about the 780i and 790i is they often require the user to jump through hoops to install Vista on a RAID setup. Despite this, the old southbridge and SLI is not the hands down winner of the multi-GPU battle at the moment, the 790i still holds one major advantage over Intel's offerings. The latter may have become less hostile to the overclocker, but nVidia is making tweaking your computer downright luxurious. Their nForce System Tools software offers an impressive amount of information about and control over board related items (overclocking, fan control, etc.) in a slick Windows GUI. The Enthusiast System Architecture (ESA) takes this a step further: components meeting nVidia's requirements will receive ESA certification and these parts (cases, coolers, PSUs, you name it) will also provide information that can be managed from the nForce software.
With the X38 available for $200, the 780i ranging from $250 to $300 where the X48 boards start, and the 790i calling for another 50 bucks, taking you all the way to $350, the X38 appears to be out in front when it comes to bang for your buck (assuming you get one of the many X38 based boards that support 1600 FSB chips), especially when you figure you can throw one or two (or more) Radeon 4850's in there and you have a very solid gaming rig.
At the top, it becomes harder to pick a clear winner. Some people, especially if they are going to buy a $350 plus board, have every intention of absolute GPU supremacy irrespective of cost, in which case SLI still beats CrossFireX. These people will also go bananas for nVidia's tuning software. The recent Radeon release does however create an opportunity to get almost the same performance for a lot less money by taking the X48 route, all while getting the piece of mind and trouble free running that an Intel chipset usually delivers. That is something a lot of users, even in the super high performance market, will prefer, to the ability to tweak every little bit of their system.
Find out what kind of chipsets are out there for AMD CPU's or skip to the Conclusion.