I currently use an AMD 4850 with the catalyst driver, however I now ordered a nvidia card.
If I switch the cards and boot my pc, what do I have the expect? Do I have to uninstall the catalyst before using the nvidia card or will the kernel simly fallback to an vga driver?
I switched from Intel to nvidia.
After putting the card - on boot up choose the fallback option of grub menu - it gets you a working desktop.
Have a read of https://wiki.archlinux.org/index.php/NVIDIA - follow this nvidia wiki.
Get conky weather on your desktop https://bbs.archlinux.org/viewtopic.php?id=142640
I would uninstall the ATI drivers, install the NVIDIA ones, shutdown, switch hardware, boot. I won't guarantee it'll work, as I haven't done it, but I see no reason why it wouldn't and you can always use fallback if it doesn't.
This isn't Windows, I'm pretty sure you can even have both AMD and nvidia. At least the open-source ones. The kernel will load the appropriate modules for the console (or fallback to plain old framebuffer), and X should work too.
There's no appropriate order to do it (just like the Arch setup, you don't have any driver loaded, and we still can intall Arch fine and fix it), but 1Samildanach's solution should result in the nvidia driver working out of the box on first boot. Otherwise, you will get a regular console in which you just have to install the missing packages and try again.
Everything worked quite well: put the new graphics card in, booted, removed catalyst installed nvidia, reboot, done :-)
The process was faster in in windows, where i had to download >200mb for the driver ;-)
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