Specs and synthetics
Before we get to play any games, we should stop and look at the specs of the various cards we're testing. Incidentally, the numbers in the table below are derived from the observed clock speeds of the cards we're testing, not the manufacturer's reference clocks or stated specifications.

The theoretical numbers in the table give the GeForce GTS 250 a clear advantage in texture filtering rates and memory bandwidth, while the Radeon HD 4850 has an equally sizeable lead in peak shader arithmetic capacity. But look what happens when we run these cards through 3DMark's synthetic tests.

The 4850 1GB nearly matches the GTS 250 in the color fill test, which tends to be bound primarily by memory bandwidth, and the 4850 comes out on top in the texture fill rate test.

Meanwhile, the GeForce GTS 250 leads the 4850 in half of the shader processing tests, and our expectations are almost fully confounded. In this GPU generation, the theoretical peak capacities of the GPUs take a back seat to the realities of architectural efficiency. Although the G92 has more texture filtering potential and memory bandwidth on paper, the HD 4850 is stronger in practice. And although the 4850's RV770 GPU has more parallel processing power than the G92, the GeForce tends to use its arithmetic capacity more effectively in many cases.