Let me first say welcome to the MythTV community. I hope MythTV is as good an experience for you as it has been for me.
My first question back to you is - have you had any recent experience with Linux? If you haven't you may want to install a standard distribution of Linux, like Ubuntu and just tinker around a bit just to get yourself a little familiar with how things work in Linux, before you dive into MythTV.
I think your decision to go with a pre-packaged version of Mythtv (i.e. MythBuntu) is probably a good idea. These packages are usually a little less difficult to get operational and usually limit your ability to mess things up with incompatible repositories and a bunch of stuff to difficult to explain within this post.
One other thing; You should dump your ATI card and replace it with an Nvidia card. The linux driver support for the ATI cards is not that great and in general you will have a lot less problems with your system especially after MythTV is added to the mix, if you go with an Nvidia graphics card. It is not necessary to get anything outrageous or expensive, anything from the 5200 series up should work just fine. I like the 6600gt myself, cause you can get them cheap and fanless.
Hope this helps & have a great time,
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