as well, i should note, if you follow my guide, it wont remove anything from your existing install, i run XBMC and mythtv on the same install, just had to add a menu item to them... and yes you can use SMB share, you just need to install samba on your installation, but i would probably do that on my backend not the front end, and then NFS share it out from the backend! (much easier, and more realiable). i have my backend pulling my download dir on my windows box to a video dir on my mythtv, they all share the same database, so when i do an update videos, it pulls all the new "stuff" from my windows box into the db, then i can watch my divx, xvid's ect....
one of the challenges you'll have with mythtv is making sure all of them pull from the same directorys (on every machine) so basically you want to make sure all your video directory are setup like
and have sythlinks to these directorys (sythetic links)
so on the backend you would have a sythlink from /myth/video to /mnt/video
/myth/tv to /mnt/tv
what i actually did was a little different... here is a great guide from the guys over at knoppmyth for a system he called LinHES: Linux Home Entertainment System.
i followed this pretty much to the "T" and it gave me a ton of ideas, i only had a few problems getting my frontend to burn dvd's from encoded shows, but that was more of a database issue since i use it as a backend as well.
if you have any questions, post away man, anything i can do to help this community out is great.. Now i just need to find a sweet deal on some X10 components and ill be a happy geek (i have none right now, and really dont know what im looking for, but ebay is being a bust, and being up in Canada, means i have to pay so much in shipping and duty - It Sucks!)