3DMark is called the "Gamer's Benchmark," and THG has been using its different versions for years. The mark is set using a reference system; the higher the score, the better the performance of that system versus the reference one. With each new version of 3DMark, scores are scaled down so they do not become astronomically high. These third-party benchmarks are widely used across the industry.
Clearly you can see that the GeForce 7800 GTX 512 pulls out in front of the ATI X1800XT with higher scores.