It seems like PCMag didn't do their homework properly, as the Falcon Northwest system they tested could've come with Radeon HD 2900 XTX 1GB cards. Falcon Northwest is listing two different configurations with Radeon HD 2900 XTX cards on it's Mach V configuration site and a single card carries a $164 price premium over a HD 2900 XT.
If this is indeed the case, then this would partially explain the huge performance advantage the Falcon Northwest CrossFire system had over the two SLI systems. A CrossFire setup will cost an additional $1,360 over the default GeForce 8600GT card that the Mach V comes with.
A single Radeon HD 2900 XTX 1GB GDDR4 has a lower price premium over a GeForce 8800GTX, as this would cost you an additonal $608 compared to a $565 premium for the AMD card.
You can find the Falcon Northwest Mach V configuration page here and in case something has changed, you can check out our screen grab below.