I personally think the ATI Radeon X800XL is better due to it's 400 MHz Core Clock. Nvidia's 6800GT only has a 350MHz Core Clock. That's extra 50Mhz has got to make some kind of difference in performance. Nvidia's card is 20Mhz faster in the Memory Clock, but big deal... Nivida's also has a ton of Features, but are they really worth it?
NVIDIA CineFX 3.0 Technology
Powers the next generation of cinematic realism. Full support for Microsoft DirectX 9.0 Shader Model 3.0 enables stunning and complex special effects. Next-generation shader architecture delivers faster and smoother gameplay.
64-Bit Texture Filtering and Blending
Based on the OpenEXR technology from Industrial Light and Magic, NVIDIA's 64-bit texture implementation sets new standards for image clarity and quality through floating point capabilities in shading, filtering, texturing, and blending.
NVIDIA UltraShadow II Technology
Enhances the performance of bleeding-edge games, like id Software's Doom III, that feature complex scenes with multiple light sources and objects. Second-generation technology delivers more than 4X the shadow processing power over the previous generation.
Superscalar 16-pipe GPU Architecture
Delivers 8x the shading power of previous generation products for screaming gaming performance.
The industry's fastest antialiasing delivers ultra-realistic visuals, with no jagged edges, at lightning-fast speeds. Visual quality is taken to new heights through a new rotated grid sampling pattern.
Vertex Shaders 3.0
Pixel Shaders 3.0
16 parallel pixel pipelines
6 parallel vertex processing engines
SMARTSHADER HD increased maximum shader instruction count to 1,536 from 160, new high-performance shader compiler
SMOOTHVISION HD improved AA and AF performance for HD resolutions, 12X effective Temporal MSAA generates the sharpest and clearest images
VIDEOSHADER HD integrated shader features to provide unprecedented support for digital and high definition video
HYPER Z HD enhanced Hi-Z buffer for improved performance at HD resolutions
3Dc provided image enhancement, acceleration technology resulting in high polygon characters and scenes
128-bit, 64-bit, 32-bit per pixel floating point color formats
Oh yes, they both appear to have huge fans...so I suppose they both run around similare tempratures..