GPU, in hardware. This makes the GPU more complex, but rewards the user with higher quality and higher performance. Multisampling works because the GPU itself is "aware" that multiple samples will be used to calculate the final pixel color."
You can think of these extra samples as extra "virtual pixels. The GeForce3 GPU has wider data paths internally so it can handle these extra virtual pixels without slowing down its standard rendering speed. In fact, the GeForce3 GPU can compute these "virtual pixels" or additional samples at full speed, with no reduction in engine performance whatsoever. These wider data paths enable GeForce3 to use the same texture data for all of the samples in the pixel and significantly reduce the memory bandwidth required to texture all of the AA samples.
The GeForce3 GPU is the first product ever to deliver HRAA at a consumer price point. The hardware multisampling capabilities of the GeForce3 GPU offer an unprecedented level of performance and flexibility, giving end users more and better mode choices for antialiasing, and helping them tap the full potential of this groundbreaking graphics processor.