SLI - Nvidia's Multi-GPU Geforce is Seeing double

Two types of SLI

SLI has received a lot of publicity and so many people have there idea's about the way that SLI works. In truth however there are two ways in which Nvidia's SLI works and they vary depending on the game that you are playing. SLI can work in either a split frame rendering mode or an alternate frame rendering mode. The split frame rendering mode gets the most attention because its the most like the original Voodoo 2 scan line interleaving.

Split Frame rendering literally splits each frame and sends half the load to each of the graphics cards. Top half to the card 1 and the bottom half to card 2. The software then pieces the image together and renders the complete frame. In Alternate frame rendering the whole frame is sent to a single card, then the second frame is rendered by the second card. This can actually produce faster results, however some newer 3D games cannot use this technology because of the way that they blend frames together for certain effects. 

Why Not have 2 GPU's on a single board?

There are a few reasons why this is not a simply option as opposed to the dual board SLI way of doing things. The first is a technical one which I shall not bore you with, but the basics are that certain logic problems with the PCI express bus  will make a dual chip option very difficult to implement at this time. There is then the issue of heat. With a single GPU requiring its own heatsink and fan, especially as these fans are having to be larger and more complex, a dual GPU on a single motherboard will cause the heat to be a major problem under heavy loads. The final reason is one of upgradeability. The great thing about an SLI system is that you can buy one Geforce 6800 now until you think it can no longer handle what you throw at it. Then instead of having to buy a new top of the range graphics card you can double the speed of yours by adding the same card into your system in an SLI configuration. By this time of course the price of the Geforce 6800 will have come down substantially. This is another reason why the Voodoo 2 was so successful.

Power Requirements

With Nvidia stating that a single Geforce 6800 will require you have at least a 480W power supply, you can imagine that the requirements for the Multi GPU system are pretty hefty. Nvidia claim that an SLI system running two Geforce 6800 Ultra's should work fine running off a PSU rated at 550 Watts. Obviously this all depends on what extra devices you have hooked up to the PSU inside your PC.

Is it Worth the Expense?

This as usual is a very difficult question to answer. I would say that buying two Geforce 6800's at the moment is not worth the expense. For what you get out of the second card today it would be seriously expensive. However it is certainly worth the expense of buying a single Geforce 6800 with the view of upgrading to the Multi-GPU system in the future. This way the price of the Geforce 6800 will have come down and the system may well be worth the money you paid for it.

As always though there will be those of you out there with money to burn who just have to have the best, and at the moment a Dual Geforce 6800 Ultra is impossible to beat. Unfortunately the necessary components are not yet available, such as the motherboard with the Dual PCI express x16 slots. This is because PCI express is still young. Also with Nvidia being a chipset maker as well as a graphics card manufacturer then you will have to expect the complete package to be available soon.