Errm, I think you will find that the nVidia 7800 GT is not quite as fast as the GTX. According to reviews I've read the GT's are the same chipset as the GTX but don't quite make the grade to become GTX. Here's an extract from the link below. TC.
I quote: "Therefore, what we have here is a GPU that has all the same features as the GeForce 7800 GTX except that it has 20 pixel pipelines versus 24 and 7 vertex units versus 8. The GT’s core clock speed is running at 400MHz versus 430MHz on the GeForce 7800 GTX and the memory runs at 1GHz versus 1.2GHz on the GeForce 7800 GTX. With the pipeline reduction and clock speed reduction, the texel (fragment) fillrate goes to 8 GigaTexels/sec versus the GeForce 7800 GTX’s 10.3 GigaTexels/sec. What this means to gamers is simply that the GeForce 7800 GT is a slower version of the GeForce 7800 GTX..."