I have a development and leisure machine where I run 32-bit Ubuntu because of difficulties in the past with x64. A different system which is used for living-room surfing, gaming, and entertainment has been running Windows XP Pro x64 for some time now (which has complications of its own), but I wanted to give Ubuntu a shot. This system has an ATI Radeon HD 2900 Pro inside. I installed Ubuntu 9.10 x64 last night and it worked great, but every time I enabled the restricted flgrx drivers, the GDM wouldn't start or the screen would go black. I tried all kinds of solutions, but the only thing that worked for me was to grab the bleeding edge kernel and bleeding edge xorg which provides a driver for this card.
After doing some research, I found that the newest drivers aren't compatible with recent xorg versions, and the "older" cards like these aren't supported. I find it shameful that an expensive card a couple of years old is considered too old to support by ATI. Thankfully, the open source community is picking up their slack, and future Ubuntu releases will likely have these drivers available.