I use HDTV's for displays for Sonar too (I have a 40" and a 32"), and it all runs fine.
I happened to have had a giant pile of money when I got a lump sum disability check that went back for the 2 years I had to wait for the process to conclude, and THAT was how I got both an i7 CPU and 32 GB of memory, among other things, like my super nice recording mics and a good interface, etc.
8 GB of memory should be fine for most, if not all, of your projects. I used to run with 4 GB just fine, back with XP Pro, then moved to 8 GB for a couple years, still fine, and am now at the 32 GB.
I have another desktop I use when I go to my son's place to record him playing guitar, and that desktop is running X3 and has 8 GB of memory and is fine for anything I have ever done there. That PC has an AMD CPU of some sort - quad core but cannot recall which one.
I think you will be fine. More is more, and many times better, but can just as easily not make much of a difference, once you DO get to where you have a decent processor and 8 GB or more of memory. That seems to be a bit of a milestone amount of memory - enough to be comfortably running Sonar with a good number of tracks and plugins.
If you DO find some projects are straining things you can freeze things and such, and should still be OK.
Bob Bone