Hi there.
My x86 machine is 100% Ubuntu (7.04)
My sugestion is to leave everything as it for multimedia.
Movie player may seem the same like the 6.10 but it is lots more stable now, and will ask to automatically download any new pluggin if it doesnt already have.
As a music player, I use the mighty xmms with an equalizers file (put them in .xmms dir) with all the pluggins installed.
I dont recommend to install amarok or any kde packages. Or even the program used to rivalize with msn search or google desktop search (beagle!). If you need them, you bet start using windows and kopernic. It's better.
Ubuntu is my fave 386 OS. In my 386 computer, even the real debian distro uses a lot more resources than ubuntu. On my PC, Ubuntu starts with 100mgs ram vs Debian 150 mgs vs XP 250mgs (will never tried vista or mac os. never liked them.)
If you want a superb alternative to vista multimedia, you can try sabayon linux, or even perhaps the new Amiga OS 4 wich seems very beautiful.
About the x64 ubuntu, i think theres some repositorys add-on to make it full 64bits working. Just make a simple search around google.
If you want Amiga Emulation, just get the e-uae repository. I found E-UAE 100% stable and helped me a lot to configure my 4000. But for playing games and demos, WinUAE is better. At least in my computer. Anyway, Amiga is still the most difficult computer to emulate, comparing for example with KQEMU running smoothly any windows/ dos OS. AmigaSYS runs fine with e-uae.
I dont use beryls, etc - It's nice, but boredom after 5 minutes.
To conclude: you can safely delete a lot of asian fonts, and other stuff, like the gnome games, tomboy notes, some python help coder docs, etc without making no harm at all to ubuntu. Serpentine Audio Player is also a thing I imediatelly delete after a clean install.
This will make your ubuntu a bit more solid and faster.
I found also ext3 lots better and faster than ntfs. Lots of www-pages dont say that, but what counts is the real thing. 0 errors yet - in 2 years of Ubuntu. Youll see what im telling if you run xf dir-manager and go to a 6000 files dir... youll wait 1 sec. with nautilus is slower.
Another good programs:
easytag
Cheesetracker / Soundtracker
Liferea to see Amiga.org RSS feeds

checkmp3 - no x... (((
gnomebaker (write cd/dvd, etc)
Im telling you that after a lot of trys. You can put xubuntu and kubuntu together with ubuntu. The system keeps going, but ... what for? Also found having wine/ qemu a waste of time. Dont really see any need to have microsoft software. And samba is good, but youll have your system more vulnerable.
Oh! and avoid chmod with sudo. Also vmware if you really dont need that. Will install a v-server at startup ...
Before you install thunderbird, keep a look at evolution mail

And as you will probably try lots of things, a good thing is to put a gnome applet in the task bar called kill-window (xkill). Right click task-bar, add -- etc
Before any critics - thats my oppinion

Regards from Portugal
(But Amiga OS is better.)