Hi, i'm also a Fedora 10 64bit user and i think i might answer a few of your questions.
1. I'm not sure if ATi fully supports Linux yet, but generally the plymouth also won't work on some NVidia cards too, however a simple trick can help you here, add 'vga=0x318' to the kernel line at grub menu, this will make vesa handle the boot graphics and you can see the boot graphics. Also 'vga=ask' will generally tell the resolutions your card can handle, add the one relevant. If all goes well, permanently add it to /boot/grub/grub.conf
2. Check
http://rpmfusion.org/Configuration for RPMFusion repo installation. Once all setup, you better install 'yumex' which is a GUI to yum, then search for and install all things with 'gstreamer', 'ffmpeg' in their names. Thus all codecs and stuff will be installed. Once you start yumex, it is quite easy to search and install can can browse the packages through groups, repos, etc..However another important package for reading dvds called 'libdvdcss' isn't available in RPMFusion due to legal issues in US. It is still there in the old Livna repo. You can either add that as a repo too following
http://rpm.livna.org or manually download and install from the same site.
3. Many guides are available, however,
http://fedorasolved.org/ and
http://www.mjmwired.net/ should get you started.
4. I'm not sure as I never bothered with ATi, use yumex again.
For the RAM thing, I think it could be because of the new things(fresh code) that Fedora implements constantly in the distro, eg., Xorg in Fedora is much newer than Mandriva. It will take time to mature.