You probably won't need a Phenom X3 for a HTPC unless of course your video editing or transcoding.
A Athlon X2 5050e will work fine, but if you wanted more upgrade options I think that the Athlon II 250 is a good choice too.
The RAM just like suggested above can be changed to something like G.Skill RAM, since the XMS2 is gaming RAM, IIRC. A 700W PSU is not needed, a high quality PSU i.e SeaSonic, Antec, Corsair could easily power a system with a Athlon II or a 5050e with 400W.
Switch the motherboard to a 790GX chipset instead of a 790X so that you dont need a graphics card.
Will the onboad graphics card (ATI Radeon HD3300 graphics built-in 128MB DDR3 1333MHz SidePort Memory) support a HD 1080 LCD TV & Bluray?? I was also thinking about running Ubuntu. I have been told it is better to run nVidia due to the drivers. Is this true????
Yes, the onboard graphics on AMD boards if often recommended for HTPCs as it is capable of full HD and Bluray playback. I dont know about ubuntu support but the differences in performance between the two does not tend to be significant.