Exclusive ATI's Radeon X1900 is a darned fine graphics chip but while it has taken the fight to Nvidia's GeForce 7900 GTX in no uncertain terms it also produces plenty of heat. When ATI updated the Radeon X1800 core to produce the X1900 it increased the number of pixel shaders from 16 to 48, and in the process it raised the transistor count from 321m to a phenomenal 380m, with the core covering an area of 352 square millimetres. As a result, an X1900 graphics card draws about 150W from your power supply almost all of which ends up dissipated as heat...
By contrast, when Nvidia moved from the GeForce 7800 GTX to the 7900 GTX it reduced the transistor count from 302m to 278m thanks to optimisations that shortened the length of the pixel pipelines. It also shrank the fabrication process from 110nm to the same 90nm that ATI was using and as a consequence the die area fell from the 7800 GTX's 334 square millimetres to 196 square millimetres. The 7900 GTX's power draw is less than 100W. We're not sure how much waste heat a 7900 GTX has to shed but we'd estimate that it's about 50W - half that of an X1900.
The transistor count and die area have a direct bearing on the cost of the graphics chip, but the power requirement and heat output affect the cooling of your graphics card and PC, as well as the type of power supply that you have to install to keep everything running sweetly.
You see this difference quite clearly when you lay a GeForce 7800 or 7900 next to a Radeon X1800 or X1900. The 256MB versions of the 7800 and 7900 use a single-slot design that sheds waste heat inside the PC case and as a result are fairly quiet in operation. Your PC should be able to handle the heat without too much trouble. The 512MB versions run at much higher core and memory speeds so Nvidia fitted a huge heatsink that uses four heatpipes to vent heat from the copper spreader plate out through a vent in the bracket. This could potentially be quite noisy - hands up anyone who remember the FX5800 - so Nvidia has used an enormous 80mm fan that turns so slowly you can practically count the blades on the fan as it rotates. In operation, a 512MB 7900 GTX is effectively silent.
While a mid-range ATI card can be used on single-slot board, the performance models use a double-slot design. The heatsink is covered by a duct, and as the fan rotates it draws air from inside the case and passes it through the duct and across the heatsink. It is then expelled through vents in the bracket and is at best noisy and at worst rather annoying.