The 8800GT in my XPS 420 was too noisy. The card was fine at idle, with the fan running at 20% the temp. was about 65C. When being worked hard though (Dawn of War II), the GPU would rise to the mid 80's, triggering the fan to run at 100%. This was unacceptably loud.
I have replaced the GPU cooler with a Thermalright HR-03GT, fitted with a 92mm fan. I was concerned this would not fit, but decided that something had to be done so took a chance! This is fitted in the 'backwards' configuration, so with the BTX format of the XPS 420, the cooler hangs below the card. This means I cannot use any of the PCI slots below the PCI-E, but luckily there is one PCI slot above the PCI-E for my Soundblaster card.
I had to remove the blue bracket which previously supported the graphics card as the heat transfer pipes fouled against it. This bracket always seemed a little unnecessary.
Now, the card idles at 39C, and rises to around 52c under load (2 hours of DOW2). I run the 92mm fan at full speed (12v), but it is still only just audible even with the case open. At lower speeds, I really couldn't hear it at all, and I doubt that the temps. would rise much. As my case sits under my desk, I decided it still wouldn't be audible and left it at max.
I used the standard TIM that shipped with the cooler, and the standard RAM heatsink adhesive pads. I did make sure I'd cleaned the original TIM off very carefully with a dedicated TIM cleaner first though, having heard horror stories of these falling off - not so much of an issue though in a BTX case of course, where gravity is on your side.
(All temp. figures from RivaTuner, btw.)