An interesting find indeed.....read on.
SLI Performance Throwdown: GTX 280 SLI vs. 9800 GX2 Quad SLI
We had two GeForce GTX 280s on hand and a plethora of SLI bridges, so we of course had to run them in SLI. Now remember that a single GTX 280 uses more power than a GeForce 9800 GX2, and thus two of them is going to use a lot of power. It was so much power in fact that our OCZ EliteXStream 1000W power supply wasn't enough. While the SLI system would boot and get into Windows, we couldn't actually complete any benchmarks. All of the power supplies on the GTX 280 SLI certified list are at least 1200W units. We didn't have any on hand so we had to rig up a second system with a separate power supply and used the second PSU to power the extra GTX 280 card. A 3-way SLI setup using GTX 280s may end up requiring a power supply that can draw more power than most household circuits can provide.
Although a single GeForce GTX 280 loses to a GeForce 9800 GX2 in most cases, scaling from two to four GPUs is never as good as scaling from one to two. Thus forcing the question: are a pair of GTX 280s in SLI faster than a 9800 GX2 Quad SLI setup?
Let's look at the performance improvements from one to two cards across our games: http://www.anandtech.com/showdoc.aspx?i=3334&p=19